Also, in the beginning it was not unusual that the only portrait of a person was post-mortem - to preserve the memory (as in 100 years of solitude). Difficult to smile in that case.

However, photographs were serious business for quite a while. You don't see people smiling on official portraits for the same reason. Imagine that portrait of Napoleon on a horse, but with a smile - doesn't have the same ring to it, does it?

Same for early photographs; they were expected to be solemn.