why does it hurt to smile?

Whenever I perform a concert with Guy Few or solo with orchestra or in the recording projects that involve orchestra or just me and Guy, my face hurts from smiling and it is always a surprise.   I am not aware until after the show/session that my muscles are more tired from smiling (higher on my cheeks) than from playing.
Does this mean I am happiest when playing?  Or that I am a grim bastard the rest of the time??