Why does he want me back now? What is the motive for this?

So me and my ex0 broke up about 3 months ago. He started to see someone just 3 days after we broke up. We still talked and stuff even after we broke up. He told me he wanted me to move on. So I started seeing this guy. Well one night, we went out and just so happened to run into my ex and his new GF. We didn't talk or anything. He just kept staring at me. He and his Girlfriend got into a huge fight and left. The next day he called me up and told me he missed me and wants to try again. I asked if he was still with "HER" and he said yes so I told him "NO". He tells me when ever we see each other that he is miserable and he misses me and wants me back. I tell him I'm seeing someone and he is still seeing "her". I mean we have kissed since then but I pushed him away and told him no we can't we have other people to think about. It is to the point that I can't be alone with him because whenever I am he tries to kiss me and even go further. It isn't that I don't miss him and would take him back if I wasn't seeing anyone. I do still have feelings for him but I also have feelings for the guy I'm seeing too.

Why is it now that I'm seeing someone else he wants me back? Why won't he break up with her? If, he really wanted me back you would think he would, right? Is it that he is trying to play a head game? His friends and family tell me that I'm all he talks about and how miserable he is with her. So I don't know. I feel bad that his new relationship isn't going well, I do wish him the best but what am I to do? Please help.
Why does he want me back now? What is the motive for this?
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