First off, I have an unbelievable girlfriend, but it's always a good ego boost when somebody checks you out.
I attribute it to my looks because nobody really approaches me or talks to me. I get "Awww!" and "You're like a teddybear" (which is what people say about fat people, but I'm 5'8" and 155 pounds), which I don't consider compliments.
Don't say "It's your smile", because that's... obviously not a real smile. Hair? Flannel? Height? I'm lost. My head would especially be spinning if I happened to be single. Woo-wee.
more often than not when people feel like they usually got unnoticed or under-appreciated, it's more an issue with yourself rather than with others. Are you not all that confident? I've learned that the saying 'reality is what you make of the situation' is really true.
if you go about very self-conscious, people will probably notice and it's just human nature to pick on the 'underdog' so to say. try acting almost a little too sure of yourself, and hopefully that'll help turn things around. :)
The teddy bear comment comes from your cheeks obviously. You have teddy bear cheeks, the kind that old people would pinch if you were small enough for it to be appropriate lol. It's not an insult to be cute by the way. I'm guessing you prefer something like "hot" or "sexy" or "handsome"?
I think you're cute. You don't look massive so it isn't your weight. You've got a baby face, and that's always a plus. Mixed with the fact that you're shy, however, I'm guessing is the turn off. Girls like confident men for the most part. No worries though, you found yourself a girlfriend.
Physical looks is to a good presentation that will leave a lasting impression. I am guessing you are probably easy on the eyes for most girls so that is why they look at you. If you are not too geeky, not too much of an A**-hole, then you've got balance and balance is what most of us aim for.