It's mysterious how a camera can bring out a side of us. A side most people are glad to see.
We compose our best countenances, wear the best smiles; however, there is a great irony if you want to go down to history.

The first photograph ever shot, the 1826 photo View from the Window at Le Gras, took a whopping 8 hours to expose. When Louis Daguerre introduced the daguerreotype in 1839, he managed to shave this time down to just 15 minutes. This was a revolutionary breakthrough for photography, but still not good enough for smile-friendly portraits The invention revolutionized landscape photography. Portraits, however, were still a nuisance. Good luck getting kids to sit still for family photos, let alone convincing adults to stare motionlessly for 15 minutes. So, photographers set up some simple rules: No talking. No adjusting. No sneezing. And, just to be safe, no smiling.

By the mid-20th century, photography had become much faster, much cheaper, and much more casual. People were also taking better care of their teeth. Still, it took some time for “Say cheese” to emerge — photographers actually started a different food: prunes.
Instead of telling subjects to say cheese, photographers in British studios apparently advised them to say prunes, which would lead to a tightening of the lips. (Loll the duck faces the ladies do now.) Then, in the US, Kodak started making cameras which ordinary people could afford to buy.
It was a tough sell. Kodak had to circulate how-to books and tell people to just click the shutter, and its technicians would do the job of developing and processing the pictures.
Partly culled from petapixel.com

Nice info