In the unfriendly, anonymous environment of a big city, friendliness sticks out. People who smile when they pass you in the street are outliers.
Understand how disarming a smile is. You don’t have to walk around with the sole goal of being friendly, but know how compelling friendliness can be and how good it feels to be on the receiving end of a warm gesture.*
*I know little about living in a big city, so I can’t say that you’ll live a better life by smiling at random people on the streets of NYC. In fact, I imagine that might be a bad idea. The point still stands—a smile is disarming and special.