Who knows really, but there is something different about their brains, that's for sure. Probably like most things, a combination of genetic and environmental factors.
I like to drink sometimes, but I always stop at one or two because I know I hate feeling out of control, dehydrated, and I get headaches. That knowledge is enough to stop me. My AH, on the other hand, can drink so much he gets stomach aches, blood in his stool, throws up, gets the shakes, and feels generally terrible and it doesn't slow him down one bit. Obviously there is some sort of mental illness there.
He also suffers from terrible anxiety and has a generally negative attitude toward life. I don't know if that's why he started drinking or if he developed a bad attitude because of the alcohol. Probably a downward spiral, really. Bad feelings cause drinking, drinking causes more bad feelings, etc. It's a sad way to live.