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