Why is he doing this?
Because he's an addict, and this is what addicts do.
I know, I know. Sounds too simple, and it's probably not what you want to hear. But what I would encourage you to do is, instead of focusing on what he's doing and why he's doing it, focus on your choices.
As I said in my original response to you, your boundaries are very permeable. You need to ask yourself why you're allowing him to still be in the picture. "Because you love him" does not qualify as an answer.