I'm sorry you are feeling the pain that goes along with being involved with an alcoholic. I gave up some time ago trying to understand the "whys" in my relationship. I realized there is no understanding what is irrational behavior. It just doesn't work.
You might want to consider AlAnon or therapy to help you understand yourself. I found therapy very helpful in beginning to understand why I got into and stayed in a relationship with my STBXAH.
You cannot change him but you can change and help yourself.