Perhaps it was just the fact that "no one else smiles so I'm not going to smile and look silly".
That's pretty much exactly it! The "no one else" they were thinking of, though, was hundreds of years of painted portraits that were preserved as family heirlooms. They were emulating the only precedent for photo portraits they had in their culture (their only frame of reference, yuk yuk) - one that was about preserving your likeness for generations to come, and one in which appearing frivolous was frowned upon. So at first their expressions were culturally determined and reinforced by the long exposure times of available tech. Later - as shutter speeds increased, cameras became cheaper and easier to use, and photography moved out of studios - they loosened up a good deal around cameras. There are quite a few photos of smiling from after about 1890 or so.
That's pretty much exactly it! The "no one else" they were thinking of, though, was hundreds of years of painted portraits that were preserved as family heirlooms. They were emulating the only precedent for photo portraits they had in their culture (their only frame of reference, yuk yuk) - one that was about preserving your likeness for generations to come, and one in which appearing frivolous was frowned upon. So at first their expressions were culturally determined and reinforced by the long exposure times of available tech. Later - as shutter speeds increased, cameras became cheaper and easier to use, and photography moved out of studios - they loosened up a good deal around cameras. There are quite a few photos of smiling from after about 1890 or so.