Why do guys always stare at me?? It's annoying?

When i'm walking in the hallways at school, guys usually look at me a few seconds longer than they should. For example, I was in the lunch line and this guy turned around 3 times to stare at me for a few seconds. I find it annoying and rude. I'm definitely not ugly but I have my insecurities so I kinda take it the wrong way. I don't understand why they don't mind their own business. I'm not actually a bi*chy person but it's just one of my pet peeves. So what are some reasons YOU stare at someone? Thankkkss.