You know I have wondered this. It seems like if you talk to a guy they automatically think you are interested in them. But also, I don't think it's every guy. Some guys think that no girl is ever interested in them! But in the same way, I have flat out had guys tell me that if they didn't think I was 'hot' they wouldn't be friends with me. That they're only friends with girls they find sexually attractive but I don't think that is every guy either.

I think some of it depends on their age cause I have noticed it less the older I get. Freshmen guys in college think every girl wants them but when you meet seniors or grad students, that's less frequent. Maybe it's just wishful thinking?

I asked my boyfriend and he says that its because guys want to feel like they're wanted. He said especially if they haven't been with a girl in a while they want to think that someone likes them which makes sense to me.