Why don't people in old photos ever seem to smile?

Well besides the fact that people could not possibly stay still for the exposure time (up to ten minutes), photographs were once a serious matter that illustrated ones prosperity.  If you were to only have a few pictures taken in a lifetime, you wouldn't want a silly smile that can discredit your authority.  Additionally, people during this time did not have access to great dental care.  Therefore, showing off their teeth wasn't a good idea.