Why do we always smile in photographs?

Why do we do it? Say “cheese” and grin?

Which photo’s do you prefer, those natural ones, of people doing natural things, including yourself? Another question got me thinking of this. Face Book for example, why does everyone look so happy, and so busy, and living such full, great lives?

Would you prefer a more realistic profile. Would you find that more interesting? I know I would. Also just life photographs, caught in a moment as it really was. Maybe that is why the past seems so great, it looks so good. Any thoughts?