Why do we do things to spite the ones we love?

I recently came out of a 3 year relationship. We had our problems and I did things very hurtful to her. When things like this happen we either try to blame the other person for everything or we try to take on all the blame ourselves. I took the blame. I realize now after a few days apart, and some serious heartbreak, that even though I did alot to cause this, that she wasn't completely blameless in it (not many of us are). Why do we do things to spite the other when they hurt us? Do we really think it will make things better. It usually backfires and goes the opposite way we intended and of course most spite is done when we're mad. I've done this so many times. If I felt hurt I did something in response for spite that was equal or worse to what they did to me. Then they do something in spite again. Me and my ex spent alot of time doing stuff to spite each other. What makes us do these things to the people we love and why do we think it will help?