I can explain it in a simple way.....he doesn't live in gratitude...yet. That's the key to recovery.....living in gratitude. That is the result of working the 12 steps. I doubt he's completed all of those steps.
I, too, didn't know how to just wake up and appreciate my life until I had worked the 12 steps of Al-Anon. Then, I stopped even wanting to look at what all was wrong and started falling in love with all that was right.
Transformation of me.......through the 12 steps.
Do you attend Al-Anon, btw? Is this just "his" problem? I'd suggest you get your own program. That way, his moods can be his moods and not ruin your day. You can have a wonderful life learning to let other people have their own feelings and their own attitudes and not have it affect you so much.
I learned to do this in Al-Anon. Saving you a seat!