I think I'm starting to acknowledge that there are (in a simplified example for this post) atleast two mental states, and one is the conscious and one is the conditioned. The conditioned is the one that, if you happened to get beat up by a guy with a moustache when you were ten, would cause you to seemingly inexplicably be nervous around people with moustaches. And I just say that to mean that it's possible you do have some underlying anxiety issue for whatever reason that is in the nature of your personality. BUT thats not an excuse, I say that part just to say don't be discouraged or feel helpless when you fail to interact with someone or something of that nature. Just acknowledge that it may be difficult to get your brain out of this habit of acting because it's a habit basically.
Do you have trouble keeping up a social interaction with someone once it's already initiated? Which is to say if you finally go up to this guy and say "hey hows it going?" are you going to second guess everything you say to him or feel nervous about the conversation?
If not then really it just amounts to not looking down when he passes by and waving and saying hi. Simple as that, nobody can help you any better.
If you are just generally anxious then I'd say you should REALLY look at yourself and not lie to yourself and say "why?". Because there is a reason. You're either nervous of what they think of you, nervous they'll just be mean to you, or something of that nature.
If you're nervous of what they think of you then you just gotta stop thinking of people in that way. Like that what they think of you even matters. Really the whole point of interacting with people is to find people that fulfill you emotionally or interest you intellectually socially. You're sort of an island onto yourself, nobody else has to deal with your memories or thoughts or who you are or how you're life plays out, so if you talk to somebody and they don't like you what does it matter? If you were being yourself (not contriving the WHOLE thing to please them, it's inevitable that people contrive some things though for "politeness") then all it means is you guys don't mesh, theres no point in being upset over that or feeling anxious over it later.
But of course thats only if you feel anxious about what people think of you, you didn't give to much info in the OP.
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ROW ROW FIGHT THE POWAH