I just feel that the world is in a bad enough state without everyone being miserable/insular or ultra defensive as well, I don't go around the supermarket waving at folk, but do try to look amiable, and respond to people around me in a positive way.
I travel in France a lot, over there it is considered to be polite to acknowledge people as one passes them in the street.
Unfortunately in the UK , the south in particular, people are very reserved sometimes to the point of rudness.