This is something that occasionally pops up in my head. You walk in a relatively crowded place and everybody ignores you. But when I walk in a park, on the sidewalk or other similar places and meet someone I automatically get the urge to look him/her in the eyes and nod or say something. If I don't I feel a bit guilty for being "rude". Which is a bit silly as we wouldn't greet each other if it was crowded.

Is this because we are learned to greet each other, but when it is crowded there is just no beginning to it (the scene of Crocodile Dundee in New York is a good example of that), so we do it because it is polite. There are times that you say "Hello" to feel assured as you usually have little to fear from someone if he greets you. Or is it something like dogs always greeting each other, a behaviour build into us but that gets surpressed by our logic when there are too many people around.