Disclaimer: All characters in this work belong to Joss Whedon....um, sort of.
Rating: G, of course, like any good fairy tale
That is a good question, isn't it? I can tell you the story, if you like.
Once upon a time there was a beast. In fact, there were many, many beasts, but this beast was the worst in all the land. He was worse than his mother, and even worse than his grandfather who was always trying to destroy the world. He loved to brag about how vicious and evil he was, and so we know it must have been true. The trouble with the beasts, really, was that they were so horrible that even mirrors could not bear the sight of them, and would refuse to show their faces. And so it was that the beasts never needed to look themselves in the eye, which might have taught them something about themselves.
The Fair Folk saw the beasts, and how they terrorized humans wherever they lived, and were dismayed. And in particular they looked upon this most evil beast as he killed a particularly beautiful and good-hearted girl. This was more than the Fair Folk could bear, and so they whispered in the ears of the girl's family and taught them a terrible curse, a spell which fashioned a magic mirror into which the beast could look and see his own face, and which followed him everywhere he went.
For the first time the beast looked upon his horrible, horrible face, and it was a torment to him just as it had been a torment to others. He fell gasping to the ground, stricken, and everything that he had ever enjoyed became a stabbing pain in his memory, for everything that he had ever enjoyed had been the suffering of others. Though they cared even for the beast, and meant him no harm, still the Fair Folk felt themselves constrained to make him suffer for his crimes.
A hundred years went by, and the beast cowered under the lash. He feared even to move, lest he do some poor soul harm. He spoke often of how evil he was, though of course he no longer meant it as boasting. And in time the Fair Folk said to themselves, "Perhaps he has learned something, though he is only a beast." And they showed mercy to the beast, so that the beast met a beautiful girl. At first the girl was wary of the beast, and the beast wary of the girl as well, but slowly they grew to love one another. And at last the girl kissed him, and the mirror shattered, for the curse was broken.
For the first time in a hundred years, the beast knew no pain, and he capered about with joy. But in his joy, he forgot all that he had seen of himself, and his capering brought death to everyone who came near.
The Fair Folk were grieved, and said to themselves, "He is only a beast, and has learned nothing." They showed the girl how to repair the mirror and restore the curse, so that the beast saw again his face, and all that he had done etched upon it, and agony wracked him, body and spirit. His heart broken, the beast left the girl, lest she pity him and break the curse again. And in a distant land the beast became a hero, fighting not to do harm but to protect the people around him.
But something within the beast was never whole again.
"It is a pity," said the Fair Folk, for they truly wished no harm even to the beasts, but they could see no other cure for the wicked, vile, disgusting evil that lived in the beasts' hearts but to punish them severely, and even that did not seem to work as well as it should, so that the Fair Folk were constrained to whip and scourge the beasts, though it hurt their beautiful pure hearts to cause such pain.
Now it came to pass that the grandson of the beast envied his grandfather, and came to the home of the girl his ancestor had loved to gaze upon her, hoping to steal her away, for he hated everything that his grandfather had become. And the Fair Folk looked upon him and said to themselves, "Let us try again." They whispered in the ears of the wise men of that land, and the wise men fashioned a new curse, more terrible than the first. This new mirror showed the second beast his image, too, and yet not his image--for it was distorted even beyond what he had made of it.
And the beast cowered under the lash. Yet the Fair Folk were puzzled, for though the second beast's agony was greater, he bore up under it and struggled to become a hero. "Perhaps he has learned something," they said, "though he is only a beast, and we know now that beasts are not very bright."
But one day the second curse began to fail, and the second beast turned upon the girl and sought to do her harm, though he proved unable to hurt her. The Fair Folk were grieved again, and said once more, "He is only a beast, and has learned nothing." And the second beast went away, his heart broken, and fashioned himself a mirror like the one the first beast had, in the hope that it would serve him better.
But something within the beast was never whole again.
The other beasts saw what had happened, and trembled. They looked first upon the beasts who had been cursed, and they feared lest the lash of the Fair Folk come upon them also. And their gaze was drawn second to the girl, who had become a great warrior and gathered an army around her, and they feared lest the girl lead this army against them. And lastly the girl and her army led them to look back to the humans upon whom they preyed, and they saw that the humans had grown stronger while they remained the same, and they feared that humanity itself might become an army to destroy them.
So the beasts began to cower, even though there was no lash. And a few of them began to come to the first and then to the second beast, hoping to evade the Fair Folk's scourge. But the gaze of the Fair Folk was only upon the first and second beast, whom they themselves had cursed, and they did not see what the rest of the beasts had begun to do without them. And the pity of the Fair Folk for the beasts had begun to fade. "For," said the Fair Folk, "they are only beasts, and no matter how we punish them, they learn nothing. We have proven it. If we let them live, sooner or later they will strike at what they fear, and many humans will die."
So the Fair Folk began to think and plan, for they had determined to make an end of the beasts, every last one. Yet they could not bring themselves to strike at the beasts with their own hand, for, so they said, to reach out and exterminate an entire race of folk--even beasts--was itself an act worthy only of beasts. So one said this, and one that, until at last they determined to make the beasts the instrument of their own demise, for the beasts were, obviously, vicious creatures who could easily be made to strike at their own kind. "In this way," they said, "our hands will be clean." So....
What's that?
Little one, that is the story I have been telling you. Do you not understand?
Perhaps you are a changeling, child--one of the Fair Folk yourself. But if so, all the more important that you hear. I will teach you, if I can.
The Fair Folk no longer look in the mirror because, in their deepest heart of hearts, they fear what they may see.
For, my child....the Fair Folk have learned nothing.
Rating: G, of course, like any good fairy tale

That is a good question, isn't it? I can tell you the story, if you like.
Once upon a time there was a beast. In fact, there were many, many beasts, but this beast was the worst in all the land. He was worse than his mother, and even worse than his grandfather who was always trying to destroy the world. He loved to brag about how vicious and evil he was, and so we know it must have been true. The trouble with the beasts, really, was that they were so horrible that even mirrors could not bear the sight of them, and would refuse to show their faces. And so it was that the beasts never needed to look themselves in the eye, which might have taught them something about themselves.
The Fair Folk saw the beasts, and how they terrorized humans wherever they lived, and were dismayed. And in particular they looked upon this most evil beast as he killed a particularly beautiful and good-hearted girl. This was more than the Fair Folk could bear, and so they whispered in the ears of the girl's family and taught them a terrible curse, a spell which fashioned a magic mirror into which the beast could look and see his own face, and which followed him everywhere he went.
For the first time the beast looked upon his horrible, horrible face, and it was a torment to him just as it had been a torment to others. He fell gasping to the ground, stricken, and everything that he had ever enjoyed became a stabbing pain in his memory, for everything that he had ever enjoyed had been the suffering of others. Though they cared even for the beast, and meant him no harm, still the Fair Folk felt themselves constrained to make him suffer for his crimes.
A hundred years went by, and the beast cowered under the lash. He feared even to move, lest he do some poor soul harm. He spoke often of how evil he was, though of course he no longer meant it as boasting. And in time the Fair Folk said to themselves, "Perhaps he has learned something, though he is only a beast." And they showed mercy to the beast, so that the beast met a beautiful girl. At first the girl was wary of the beast, and the beast wary of the girl as well, but slowly they grew to love one another. And at last the girl kissed him, and the mirror shattered, for the curse was broken.
For the first time in a hundred years, the beast knew no pain, and he capered about with joy. But in his joy, he forgot all that he had seen of himself, and his capering brought death to everyone who came near.
The Fair Folk were grieved, and said to themselves, "He is only a beast, and has learned nothing." They showed the girl how to repair the mirror and restore the curse, so that the beast saw again his face, and all that he had done etched upon it, and agony wracked him, body and spirit. His heart broken, the beast left the girl, lest she pity him and break the curse again. And in a distant land the beast became a hero, fighting not to do harm but to protect the people around him.
But something within the beast was never whole again.
"It is a pity," said the Fair Folk, for they truly wished no harm even to the beasts, but they could see no other cure for the wicked, vile, disgusting evil that lived in the beasts' hearts but to punish them severely, and even that did not seem to work as well as it should, so that the Fair Folk were constrained to whip and scourge the beasts, though it hurt their beautiful pure hearts to cause such pain.
Now it came to pass that the grandson of the beast envied his grandfather, and came to the home of the girl his ancestor had loved to gaze upon her, hoping to steal her away, for he hated everything that his grandfather had become. And the Fair Folk looked upon him and said to themselves, "Let us try again." They whispered in the ears of the wise men of that land, and the wise men fashioned a new curse, more terrible than the first. This new mirror showed the second beast his image, too, and yet not his image--for it was distorted even beyond what he had made of it.
And the beast cowered under the lash. Yet the Fair Folk were puzzled, for though the second beast's agony was greater, he bore up under it and struggled to become a hero. "Perhaps he has learned something," they said, "though he is only a beast, and we know now that beasts are not very bright."
But one day the second curse began to fail, and the second beast turned upon the girl and sought to do her harm, though he proved unable to hurt her. The Fair Folk were grieved again, and said once more, "He is only a beast, and has learned nothing." And the second beast went away, his heart broken, and fashioned himself a mirror like the one the first beast had, in the hope that it would serve him better.
But something within the beast was never whole again.
The other beasts saw what had happened, and trembled. They looked first upon the beasts who had been cursed, and they feared lest the lash of the Fair Folk come upon them also. And their gaze was drawn second to the girl, who had become a great warrior and gathered an army around her, and they feared lest the girl lead this army against them. And lastly the girl and her army led them to look back to the humans upon whom they preyed, and they saw that the humans had grown stronger while they remained the same, and they feared that humanity itself might become an army to destroy them.
So the beasts began to cower, even though there was no lash. And a few of them began to come to the first and then to the second beast, hoping to evade the Fair Folk's scourge. But the gaze of the Fair Folk was only upon the first and second beast, whom they themselves had cursed, and they did not see what the rest of the beasts had begun to do without them. And the pity of the Fair Folk for the beasts had begun to fade. "For," said the Fair Folk, "they are only beasts, and no matter how we punish them, they learn nothing. We have proven it. If we let them live, sooner or later they will strike at what they fear, and many humans will die."
So the Fair Folk began to think and plan, for they had determined to make an end of the beasts, every last one. Yet they could not bring themselves to strike at the beasts with their own hand, for, so they said, to reach out and exterminate an entire race of folk--even beasts--was itself an act worthy only of beasts. So one said this, and one that, until at last they determined to make the beasts the instrument of their own demise, for the beasts were, obviously, vicious creatures who could easily be made to strike at their own kind. "In this way," they said, "our hands will be clean." So....
What's that?
Little one, that is the story I have been telling you. Do you not understand?
Perhaps you are a changeling, child--one of the Fair Folk yourself. But if so, all the more important that you hear. I will teach you, if I can.
The Fair Folk no longer look in the mirror because, in their deepest heart of hearts, they fear what they may see.
For, my child....the Fair Folk have learned nothing.