Why am i obsessing over someone i dont like at all?

I dont like this guy at all and infact i hate many things about him even his friends.He is a bit different from the other guys that i have met in some ways but in other ways hes just like any ordinary guy.He is a total jerk and big liar.He is also extremely stupid but very funny.I dont get jealous at all when he goes out with other girls but infact deep inside i feel relieved and happy.He used to like me and was infatuated with me and made me feel very special and like i was the best girl .I do get hit on by guys and i thought dating someone would make me forget him but that hasnt happened at all and they have just made me feel worse.I dont why this is happening.