Why do guys check you out and look at you from afar, but when you get close to them, they act like you don’t even exist?

I can see him looking sometimes from across the room, but when we’re within 5-10ft from each other, he’ll either act like he doesn’t see me, or look as if he’s looking right past me? He also hasn’t smiled or anything. I have talked to him super briefly, but just in passing (idk his name).