Sarah thought it was unfair she was born into a penniless family and had to work her way through college working at a diner whilst other people like Daniel who had no brains but rich parents had an easy ride.
So strong was her jealousy that it manifested itself into a physical Power which gave her the ability to swap bodies with him.
After the swap was complete she explained to him.
There was no point fighting it.
There was no point telling anybody what she had done.
There was no way to undo it.
So he might as well get used to his new life as a teenage girl living in a small one room apartment who had to work all the hours she cood to pay her way through college.
Which made Daniel extremely depressed because he did not want her life.