When I was young, I had friend that can smile eventhough he was punished. I still don't know why he can smile.
There are many kind of smiles eg. shy smile, happy smile, embarrassed smile, cunning smile. I don't know why Thais people always smile. Sometime smiling in worse situation, it means "I'm ok" not "I'm happy in this situatiom". Sometime they keep the feeling inside and smile outside.