1.)To get even with some one who in the past may of have hurt you.
2.)One of the worst of hurting you is in the heart when some one you truly love changes you for some one else thats when u cant think of anything but pay-back.
3.)When another person might of slapped, kicked, punched, etc... and you want that person to pay for it double the time they hurt you.
Like I Said...PAY-BACK is a BITCH.