Why people didn't smile in very old photographs.

The first American daguerreotype photograph was made in 1839.

Very early daguerreotypes, 1839-1845, took 60-90 seconds of sitting/standing and being totally still to capture an image. It is nearly impossible to hold a smile for a minute without the photograph blurring your mouth and making you look like an idiot. 

Secondly, let's face it, pun intended, people had bad and missing teeth, unlike today, where dentures and implants keep our smiles healthy-looking. 

The majority of daguerreotype pictures we see today are post-1845 when a new chemical process, 
the addition of bromine fumes, reduced exposure times to just a few seconds.
One of the oldest photographs of a cat, a daguerreotype from the 1840s.
President Abraham Lincoln's cat, "Tabby," gifted to him as a kitten in August of 1861. Circa 1862.
Please raise your hand if you understand this explanation.
Written by Dr. Neil Gale, Ph.D.