A vicious animal attacks you on a moonlit night. Friends drive the beast off, but you discover that you've been cursed with lycanthropy, to become a similar beast under the full moon. (work with your DM to determine details)
Divorce: During your adolescence your parents split up and you were forced to chose which parent to live with (Work with your DM to detirmine the details)
You find yourself in love with another, wither it is a fling or something more serious is up to you. (work with your DM to detirmine if it is ongoing or terminated)