I've been having dreams about a boy i used to go out with and love but I'm seeing someone else now and happy with that so why do i keep having dreams about us kissing and when we used to go out?

Well if you loved him at one point then you might have let him go in a way you weren't comfortable with and didn't really let him go. But if your happy with the guy you are with now it could just your letting your mind wander to much and it go's to those memories. Another reason is you might not like the guy you are with now like the other one in which case you should talk to the other about getting back toghether.