I have a goal. Sometimes, I want to welcome people–total strangers–with something, be it a smile, or a “Hello.” It’s not the biggest goal that I have ever had, but it’s important. I want to greet people not because I want an acknowledgement necessarily, but because I am either genuinely happy with seeing them or I feel good enough to make the gesture.
I want to move a simple “Hello” as an essentially selfish act–I am welcoming you primarily as an affirmation of myself, which is why a reply is so essential; to a selfless one–I am saying “Hello” to you because I feel really good, and want to share that with as many people as possible.
The other side is that people tend to be on guard; as if by smiling you want something from them.
This is why I don’t want anything in return.