Why don't people smile in old photographs and, well, technology is partly to blame.
The Daguerreotype, the first widely used photographic process, was invented in 1839, and the exposure time in those early days was really long, sometimes lasting up to 15 minutes or so, way too long to hold a smile.
But that timing was cut down pretty quickly, so there was more at play here than just the tech.
Although it was less expensive than having your portrait painted, getting your picture taken still wasn't cheap. Some people had just one photo snapped their whole entire life, which made the event a pretty important and formal deal.
Unlike portrait painters, photographers weren't trying to capture an instant in their subjects' lives, but more of an ideal of that person. Plus, back in the day, the classy just didn't say, cheese."
So, picture equals sourpuss.
It wasn't until the turn of the 20th century when cameras became portable and easier to use, that pictures turned into casual snapshots, and smiles became more common.
Now, we can't get enough of those pearly whites.