What is it that makes some ex's so cruel and nasty to their former partner?

Just reading the e-mail question and wondering why an ex would spend so much time and energy on messing with their former mates. Isn’t that just wasting their own time on the past? What makes the ex spend so much effort on just being hurtful? I can’t get my head around trying to hurt someone I loved at one point. Your thoughts? And as always, any good jokes greatly appreciated.