What makes a guy not wanting to anything with his ex but at the same time upset when she decides to sleep with other man?

He got rejected years back and is now single. He doesn't want anything to do with her. But reached a mental breakdown when she decides to hookup. To this day he recalls who she slept with.

What makes a guy not wanting to anything with his ex but at the same time upset when she decides to sleep with other man?
Post Opinion