Why does my ex care if I am with someone else after he left for another woman then blocked me for months? He unblocked me when he found out about my new relationship.

Why is he jealous of him? He literally left me. We dated three years btw. I just don’t understand. He told me he didn’t care about me and he never thought about me but soon as I moved on he came back. He told me his girlfriend didn’t know he was talking to me (she hates me and told him not to talk to me) and she didn’t have a phone so he wanted to talk to me to waste time. I ended up blocking him.

He said my boyfriend will break my heart one day and my ex doesn’t want me to “get hurt”. He doesn’t know my boyfriend at all. He said he just wants sex. He kept asking if I’ve done anything with him. I told him it was not his business. We were talking about my dog and he randomly asked me was I messaging my boyfriend. I was like “um yeah why?” and he said he was just curious.