I've never done that. But, its been done to me. I can honestly say, its the worst feeling in the world to have someone you love, disrespect you and take advantage of your feelings that way..
People do that when they are unhappy with the other person, and they are too cowardly to say anything to the other person. They just want to feel better about themselves, by finding someone clueless and dumb enough to fall for their lines.