I agree with duchess and MyMaria but disagree at the same time.
Seems like girls in this thread conclude like lightning, too one sided it comes to a point where it's not fair to the guys.
samantha, i'm a guy and the reason why i decided to reply in this thread (though it's hard for me coz i'm not into love advice) is because the situation you and your boyfriend is in feels like home to me, sort of.
here are the reasons why he's acting that way:
1. he's insecure: there's that gut feeling in him that something isn't just right. period.
2. he's afraid to lose you: some guys are just like that, believe me. especially when you're sexy.
3. IT IS in your personality: when you're the kind of girl who loves to hang out with guys, chances are, he wouldn't trust you that much.
these are my top 3, there may be more.
chad