He is incapable of love and responsibility, any active addict is. He know what he wants for a moment then he wants the drugs more.
It's not you, it's how addiction works and it's sad and painful and unpredictable and to think it is anything more is an illusion.
There is no answer to "why" of your questions except that he is living in the world of addiction and doesn't sound like he is ready to leave it anytime soon.
Try for one day to just focus on you, do something good for "you". There is a book "Codependent No More" by Melody Beatty that is one of the best ever written about why we are codependent and how we can change. The library may have it, or a bookstore near you. It's been around for years and has helped many of us more than we could have imagined.
Good luck.