Why is my ex so jealous?

My ex that use to see contacted me again. He went to prison for 7 months and he said it’s because I left him he went crazy attacking the police. He isn’t stable I don’t know and he wasn’t really good to me so I left. Now he came back after a year saying he loves me and missed me. So then I told him I like someone else and he’s like “forget him.” And he wants to know who and if he was better at sex than him and keeps on saying he loves me. He also keeps on saying I love him still when I don’t. And he said it’s mad that I had sex with another guy after him and how I hurt him cause of it.
Why is my ex so jealous?
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