Why is it you can't get over some people?

I'm well known for getting over people quickly. But there's this ex of mine, and we have this messed up thing were we don't talk for ages and then he comes back in my life apologizing, he wants a relationship again but i just want to sleep with him, and then he goes out of his way to hurt me in a really major way (publicly shouting abuse at me or embarrassing me on social network sites, pathetic i know) i just can't stop wanting him though. Am i just addicted to the pain he inflicts on me? Because i just don't understand why i can't stop wanting him. Is there some biological reasoning?