When a guy looks at you a moment, slowly looks away, then ignores you very annoyed?

What are they exactly thinking in their heads? I've had this happen to me two times and it makes me curious just to know. Are they thinking their not good enough or just doesn't feel any connection? Why would you look at her in the first place if you didn't think she was in your interest?

It's flirting. Looking at them. When their eyes meet, slowly look away. Guys can be mean about liking someone too though so I'm not sure