Most girls are still self-conscious about themselves and you staring at them makes them nervous. Feeling that you have to compete with Angelina Jolie or Kate Moss isn't exactly helping with growing up and changing into a woman :D.
If they have never seen you or have never talked to you, they have no reasonable reason, to hate you. They just feel nervous about the attention you are giving to them, they're uncomfortable with themselves, not you. :)
You can help the situation by smiling, for example. Being friendly. :)
i do it because I'm shy, and I especially do that if I like the guy because I don't want him to think I like him... weird I know, but I just don't want him thinking, why is she looking at me? she probably likes me lol.
I'm 17 and when I notice a guy looking at me I kind of feel awkward. I mean I have a boyfriend but I guess getting attention from other guys makes us feel good but at the same time the person doing the gawking could make us very uncomfortable.
Shyness, maybe not interested. Don't be phased with this, some will give you a response others will do this. At least your trying to make contact, next time when they walk by try adding alittle "hi" and see what happens - once again some will ignore you, others will give you a "hi" back and the few will stop and have a chat. Good luck brother..