He doesn't want me, so why is he jealous?

this may sound dumb and maybe i'm over thinking it, but a guy friend of mine asked me out and i wasn't ready. then when i was he told me he wasn't ready and we had this back and forth flirting thing going on. But he's a bit selfish and doesn't treat me the way a guy should if he liked you PLUS he told my friend that i was coming on too strong one night and forcing drinks down his throat (which is a LIE). I got over him. Yet, whenever a guy shows interest in me he seems jealous. The way he looks and acts (all odd and nervous like). Today he was with my friend and I told her I was meeting this guy for dinner. I din't want to add too much detail, but she asked the guys name and I told her. Then she said, oh yea that guy we hung out with last night. The guy i used to like heard that and his eyes got wide and he seemed upset lol I don't know if I'm imaging it or not, but w/e. Why seem jealous when he obviously doesn't want me!? He prob wants me as FWB, but I want a REAL r/s.