Between evil stares, paranoid looks, and judgmental eyes, it is nice to see someone smile as we make eye contact. That is friendly etiquette, for those that don't know how to greet someone. It's like how a handshake when meeting someone is/used to be a way to just make the interaction easy and more comfortable. So I stopped 20 feet after I walked by and told the person who smiled at me, thanks for smiling at me. They said they didn't even realize they did it. So nice to see rather than ugly, unhappy, and worried faces.