Why do guys want to hook up with me, and not date me?

Why is it every time I like someone they either don't like me..or they just want to hook-up with me. but they never want to date me? I mean lots of people tell me I'm pretty and I don't flirt a lot. I don't really unless its with the guy I like and I only focus on one guy at a time. but say I liked a guy since I was really little and then stuff happend between us and we hooked up just once. but when he is around his family and friends he treats me like I'm just a little sister. why does he do that, because I'm not his little sister and I want something more but I don't know how to approach him about it...please help me..