I only stare at girls if I am attracted to them. These guys that stare at you are probably attracted to you and that is why they stare. They may also be really shy about trying to talk to you or approach you. They may be thinking that you are so pretty that you must already have a boyfriend or a bunch of guys chasing you. I have a lot of shy guy friends who do this with girls they find attractive. They just don’t think that the girl could possibly like them so they talk themselves out of trying to start a conversation with the girl.