Why would he start dating his best friends ex?

The guy I was seeing hated his friend’s girlfriend because she was toxic and mean to his friend and she was controlling while they were together. Around the same time the friend broke up with the girlfriend my guy and I had a situation that caused both of us both to feel like we hurt each other but unintentionally and we didn’t know how to handle the situation. We were distant and he started dating his friends ex which he was still coming over to visit me nearly daily. We went from spending so much time together and I had so many people tell me that he was in love with me to him dating his friends ex which he hated I just don’t get it 1 week after ours and 3 weeks after her break up they are together? He also never told me he was in a relationship and keeps turning on and off his relationship status online he moved in with her after 2 months and at 4 months he posted saying how he loves her. Why would someone do that? He contacted me multiple times to tell me stuff happening in his life he asked me to talk with his girlfriend and he left his friend there still friends but not really

Why would he start dating his best friends ex?
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