Why do I get attached this easily and what is the point anyway?

When I start dating a guy and want to have a relationship because we seem to have se much in common, why does he suddenly say he just wants ...then what was the purpose of getting to know each other and gettin gmy feelings involved if he just wanted to have fun for a night? And then when I find out that he just wants , I say no and that I won't do it unless I am in a relationship and we agree to be friends but usually, for him this means that he has a few more opportunities to rephrase his request. At this point, I break up with him or start ignoring him and he got really jealous about me hanging out. Why would he get jealous if he doesn't want me? ALso, why do I feel heartbroken after I break it off because he just wanted (its not like I really had him in the first place)?