I knew a guy who studied South Indian dance who told me that smiling was part of the training, as in one was supposed to smile was dancing and that this not only made a certain impression upon the viewers but an affect upon the dancer that was valuable for endurance. I came upon it again in a book about mysticism where the guru was instructing students to experiment with what smiling all day did to their internal state (or for as long as they could) and I’ve seen certain musicians intuitively do something like this when they play. It later made me think about the ones who look absolutely frightening and in pain when they play. I don’t have any conclusions other than it’s noticeable how some look in pain and some look in love though you’d think the activity is supposed to be more about the bliss business.