I think that trying to figure out WHY someone else is the way he or she is, is about as useful as my trying to figure out WHY I'm an alcoholic. It's really kind of a moot point--it is what it is. What matters is what I do about MY problem--my obsession with the other person, my obsession with alcohol.
Figuring out the "why" doesn't fix anything. Despite your belief that it will, it also doesn't give you any closure or peace.
But if you are able to ACCEPT that he is what he is, and move from there, THAT'S when you start to get better. Acceptance really is the answer.
I think you might find Al-Anon very helpful, as long as you realize that it won't give you the answer to the "why". If you are tired enough of feeling miserable and are willing to take a look at your own reactions and how THEY hurt you (he doesn't have the power to hurt you unless you give it to him), you might start feeling a whole lot better.