I agree, I love watching people waiting for or getting on the bus or metro, especially if they look tense or crabby, making eye contact and smiling. First they look startled, then smile back, then perk up, sometimes look a little embarrassed as if to say, I wasn't crabby at all!.
There are a huge number of muscles in the face, all with nerves connecting to the central nervous system, when we use them to smile, it changes feeling and thinking. It sure is fun to watch too!