Because you choose to tolerate the abuse and he has a need to be abusive.
That's pretty profound.
You know meant to tell you that I've been reading a lot on co-dependency and I don't fit into many/most of the categories. I'm not sure that's the name for my self destruction in relationships. I think it's more of me being obsessive. I honestly can see myself without my XF, but I can't see him with another woman. That stings and I can't stand that pain so I stay.