The person being photographed has noticed that the instruction to smile or look happy is asking their face to move faster than their feelings can catch up. It is a small choice: perform a feeling you don’t have, or stand there honestly and look, to the group, like you’re being difficult. Hand someone a camera at a party without announcement, and the people who usually retreat during group shots often don’t. Sometimes they don’t, and the picture shows a slightly absent person standing in a row of grinning ones. That’s the picture that gets described later as you look weird in this, why didn’t you smile.