So after being on G@G for a while I've noticed this question over and over again"
"Why is he staring at me?" or "What does this kind of stare mean?"
And since then I've paid a lot of attention to where my eyes are and how it could be perceived. And I've come to a few conclusions
1- Not every look toward a girl is AT that girl
Guy's look at posters, out of windows, at designs etc. You might just happen to be between him and the object he is looking at. There won't be any attempt at eye contact and will look like he is frowning or concentrating.
2- It also might be that it's about something on her.
I've found her shoes interesting, or perhaps a neat pattern on her fingernails. It's the object that has caught my attention not the woman herself. I'm not interested in her in fact I forgot that she's even there. This often has a look of concentration with it as well. (And yes this can even include body parts, I know guys are pigs.....)
3- If he is looking at you it may be that he is interested, but it might be that he just want's something else.
Guy's are hesitant to approach women who are alone for fear of scaring her. So he might just want a cigarette, the time, directions, a turn on a piece of equipment or something else equally benign but without a "go ahead" signal from the girl, he might not approach for fear of coming off as a creep. Guy's will look hopeful or friendly. Make eye contact, but won't approach if she doesn't give him permission in some way.
4- There is also the censer look, he might have heard that you slept with his best friend and dumped him horribly.
Yes he is going to stare, and no it's not going to be friendly or polite. He's mad and wants you to know about it. This has a frown or anger look about it.
5- And of course there's the flirty looks
Looking at her, waiting for her to notice, then turning away. Then look at her again, until she notices and repeat. Another flirty thing is that look at her with a half smile and when she looks to motion to her in some way (salute, glass raise, tip a hat, etc.) And see what she does, (eye roll, smile, turn and kiss her girl friend [yes we know it's a game to make him go away]).
The thing to remember is guys are very visual creatures and relate to their environment with their eyes. Not every look "means something" and often it has nothing to do with you.
If in doubt, make eye contact with him and smile, if he looks startled it really was about something else. I hope this helps.