I think sometimes we are "surprised" when someone tells us we should smile more and in those cases, I would look at that situation; see what was going on (or not). Could just be that we are giving a "blank" look and not engaged with the person? If the person is a friend or acquaintance and we don't "greet" them with a smile, that could be disconcerting to them? Or, if they've just said something they think is smile-worthy but we were self-absorbed and didn't hear it!

The time I remember being told I should smile that got me to call the therapist right away when I got home was by my dentist!