Why guys stare at girls?

There was this guy in my year, always used to stare at me. I caught his eye many times, sometimes i saw him staring at me from the end of the lab, and sometimes from the other side of corridor. I know he liked me and i also started to like him but the only thing was that we never spoke to each other. And now he has left as he's 1 year older than me. I still have feelings for him. Why guys stare at you, does it mean they like you? and if they do why don't they ask you out or speak to you, I'm a shy girl and so i coudn't start the conversation.