Why do people hate hearing about past relationships?

This one guy gets so uncomfortable and frowns when I mentioned that I once kissed a guy a year ago or how this one ex bf use to go to the movies a lot with me. He also doesn't like it when guys hit on me. Seriously though, what's so wrong with talking about something casually and briefly (not too many details) from your past?

Why do people hate hearing about past relationships?
Post Opinion