Cos he's a boy, playing boy games. He doesn't like to see you with anyone else. Just live your life. Forget about him.
Why doesn't he just go back to her? My ex cheated on me and I left. I have a new boyfriend now and he still likes me for some dumb reason. The other girl still wants him and he's done with her.
Please no offense intended, but some of your questions are about your ex, and I don't understand that. Are you really over him? Because you sure want to talk about him or his motivations a lot. You have a new boy and you seem really happy. Why are we talking about your ex? How about we don't talk about him anymore and let's talk about what's going on with you or things that interest you? Leave him and thoughts about him in the past.
I agree with dragonfly.
You're not done with your ex.
Work on it. Or not. Some folks like and thrive on DRAMA.
It's good to leave him in the past, but I know from experience it's hard to do sometimes. The first thing to help you get past it is to focus on what you have now - your new boyfriend. Ignore the old boyfriend - pay no attention to what he's doing or not doing. In time, the feeling that you need to check on him will pass. It's a habit and as with any habit, it takes time to change it. But I have faith that you can. Good luck!