I never will understand why he does what he does.
I had an experience in which someone treated me poorly. It doesn't matter how or the circumstances, this is a life lesson for practically everyone. I was on the phone with a friend, near tears because of the frustration, and I said, I just don't understand why he'd do something like this. And my friend said:
"You don't understand because you would never do something like this."
It's really that simple. I've had people opine to me about situations they come across "The house was a mess...He's doing this...She's not doing that..." and I can say, "Mmmmm, well, uh, have you ever been diagnosed with depression?" because I know what it's like to discover that cat used my favorite braided rug for a litter box while I was grieving AH's death. I've been there. I can tell people that, and sometimes they 'get it' and sometimes they just don't. They walk away shaking their heads. It's the same thing with an alcoholic. To those of us who haven't been there, it can be mystifying. But to the alcoholic, (or the mentally ill, or the kid who going to lie about getting into the cookie jar) there is a justification and reasoning for what they're doing.
We can't get inside someone else's head, but we can accept the idea that that space is different from the inside of our own head..