@Mammy7 I have noticed that people who constantly ask "why, why, why" about other people's shit behaviour, tend to have been treated like shit as children. My mother does it - so do I. It is something I have had to train myself not to do. Because it doesn't matter why they do it. What matters is, they're doing it.
I'm imagining that when you were a little girl, your heart was broken many times and you spent a lot of time trying to figure out "why" your loved one didn't take good care of you. So that you could fix it. Because you were a child, and needed them to take care of you.
But you are an adult now. You don't need to work out why, because you aren't trapped with this person, they aren't your parent/teacher/etc.
You can vote with your feet.
Every time you catch yourself wondering "why" - gently stop yourself. And redirect your thoughts. Start asking, "what is he doing, and is it OK with me?" "what is he doing, does it make me feel happy?" If the answer is NO, then start your next line of thought: "How can I get this person out of my life and headspace?"