I used to smile wrong. I would spot an attractive woman, make eye contact with her and then smile full-tilt. My expression jumped from dour to grinning in a split second. This came across as forced and awkward and kept me celibate for years. A woman would think, 'Why is this guy suddenly smiling at me? What did I do? Is there mustard on my blouse?' I would make her feel uncomfortable and paranoid. I didn't come across as a happy outgoing person but rather as a person who put her on the spot.
You probably have had the experience of hanging out with some friends in public. Maybe it was at a restaurant or maybe a shopping mall. You were happy and laughing and smiling. But then you turned away from your group briefly and caught a stranger's eye. That was when an amazing-thing happened. Even though your smile was a random ‘drive-by' smile and not meant for that person, the stranger warmly returned it. Maybe it was an old man, maybe it was a punk rocker, maybe it was a hot girl, and maybe you turned away in surprise but the exchange felt organic.