I was thinking of sour/serious faces and how prevalent they can be.
There are the politicians who pretend to be serious/unhappy when revealing something bad about their opponents (while concealing the happy smile underneath).
There are the joggers on the bike trail who feel that jogging is a horrible way to get exercise with their sour faces as they strive to get yet another step further.
There are the bell choir members who feel if they smile they may miss the next note (I think bell choirs could recruit more members if at least one of them was allowed to smile).
My LW smiles when she sings and is a great advertisement to get others to join her choruses; others frown like "why are we here, singing is so hard" and "why are we singing this Swedish stuff in this Swedish Women's Chorus."
When I was in operator licensing and giving exams, we had to not give away how well or bad the operators were doing; so there we had to have a blank/semi-serious faces.