We would smile…
… when it is more laborious to scowl.
Of course, this is nearly always the case. Frowns instigate turbulence and propagate ill temper in our compatriots while bright eyes and easy laughter effortlessly dampen tumultuous seas and infect our companions with mirth.
I rightly chide myself for careless snapping at friends, lovers, and colleagues. In the thrust of anxious moments and world-weariness, I wrongly imagine that producing a smile is a favor that I owe them.
Producing a smile is a gift to oneself.