Comment by ordu

If Russians smile more (I do not know really), then the most likely explanation is a mixing of the USA culture into ours.

I being Russian just do not keep smile on my face when everything is just fine. I'll keep smiling when things go especially good. I'd laugh evilly^W when they go in unexpectedly good way. But small variations from a statistical average is not enough of an emotional reason to change my facial expression.

Statostical average is the key. If things made a habit of being extremely good, I'd stop smiling when they are extremely good. I'd wait for more exciting occasion.