Why don't boys ask me out?

I'm 17 and I never had a boyfriend. I get along with most students and I think I'm friendly. I never had trouble making friends & I have a lot of both male and female friends. My guy classmates or friends have never asked me out. Some had a crush on me and told me about it but they've never done anything about it. I see a lot of other girls, pretty or not, getting boyfriends. For some time I thought that I wasn't pretty enough or good enough but now I've stopped beating myself up for it. About my looks girls tell me I'm pretty and my guy friends call me sexy, its annoying but they never listen to me when I tell them to stop. Oh, the other problem is how guys act when they don't know me. They get kind of shy and don't make eye contact with me. One day I saw my friends talking to this guy from our last year's class and I joined them the guy acted like I'm not there & didn't even say hi. I know he's a shy guy but atleast hi? I don't think I am intimidating. But after the boys are past that stage with all the shyness and awkwardness we become really good friends. Some become over protective but I think its cute. Anyway to summarize this, boys, why wouldn't you ask out a girl you have a crush on & from what I've written what is it about me that makes guys uncomfortable when they meet me?