They do this because we let them get away with it. We stay and stay and stay and hope that this time things will be different, but if nothing changes (that means US not them) then nothing changes.
Read the sticky about what addicts do at the top of this forum and then ask yourself, what you are you getting out of it.
He's sick. He's an addict. You can't fix it. You can't understand it. You can only make yourself sicker than he is by trying to figure it all out.