For me there's a big difference between someone flashing their teeth in a big grin, and a photo that freezes the moment in time. The longer I look at such a motionless smile, the sillier it seems.

FWIW, I'm from Germany (way to the right on the graph) and probably somewhat of an outlier, since people kept asking me whether I was sad when I just made a neutral face. I ended up training myself to always curl my lips slightly to stop those questions.