I was out walking down Main Street with a couple of girls the other night and a car of guys hooted as they drove by. One of them complained how distasteful it was for a guy to do that. "What does he think he's gonna get?"
And maybe I'm weird... but actually I really like it for that reason.
I don't want a guy I don't know to walk up to me and start hitting on me for no reason other than that I'm pretty. That's shallow and extremely unattractive. I mean, yeah, attraction is important, and I want a guy to think I'm pretty, and it's probably what actually initialized interest, but I still also want to feel like he's interested in finding out what's BEHIND my eyes rather than staring at their surface... or at anything else.
You're driving by a girl you think is pretty. You'll never see her again. You have nothing to gain or lose by hurling universally understood messages of "I think you're hott!" It might just be the purest form of compliment.