In fifth grade, in my very first California school, I must have smiled a lot because my teacher always remarked about it. It was very nice, I should keep smiling, etc. Yes, I must have been happy at school, but you know, I had no other way of communicating since I was as shy as a snail.
I continued smiling, even as I grew out of my shyness finally in college. And later, I smiled at work, at my neighbors, anybody who crossed my path.
But in Paris, unfortunately, I've developed a locked jaw and stone face. It's not pretty, but it's just the right way to go. You fit in, you don't give anyone a wrong signal, and you don't look stupid (this last one I just learned about from an expat of seventeen years who has discussed the subject with her French friends).
I hope my smiling muscles will return when we go back to the States.