This is an interesting topic.

When I began work in a store I noticed how a high number of customers smiled at me (just as I was walking past them with store goods and thanking them for waiting aside), yes I was wearing a smile and they were returning it but I think it was the store uniform and some kind of etiquette. It's been a nice part of my job that so many people react that way.

Most people respond favourably to those small interactions, but some do not at all, and a small number can be not so nice. Human nature is fascinating - and definitely don't let those who are not so friendly put you off being friendly.