Starlight Glimmer was a specail unicorn. As in, she had special needs that made her easily angered an often picked on as a filly.
Then, after one of her bad days at school, she end up at a cave outside of her town that offers her to make the world in the way she wants it, where everypony is happy and isn't picked on. All they have to do, is give up there cutie marks and smile all the time no matter what.
Doesn't that. Sound fun?

Part of this story idea came from Dr. Wolfs theory's he had on the clip preview at commacon.