Pinkie pie was tortured on the farm. All of the darkness changed her soul.

{Pinkamena Diane Pie, a mare with a Dark pink mane, and slightly lighter coat, lived on a rock farm. She was an outcast, mistreated by her family. One day, after learning that her mother really does love her, she snaps. In a blind fury and anger, and help for her mother, Pinkamena, becomes the not-so-bouncy party pony, you all expected her to be.}
Dash is raised by thieves
Rarity works in a mine
Fluttershy was arrested for drunken assult
Applejack never returned to the farm, and became a lawyer
Twilight was unable to pass her exams, and committed suicide a few weeks later.