I believe it is a basic need to be recognized. This is my contribution:

When passing people close at hand I  give a smile and a nod. My  wife allways asks me who was that, to which I tell her that I don’t know, but I gave a nod to recognize his presence. I believe that is part of my values; to «see» people when passing.

Next time it might be you or even me.

«Always give a word or sign of salute when meeting or passing a friend, or even a stranger, if in a lonely place.» • Tecumseh