The smile in photographs didn't appear until the early 1940s during wartime when gents were glad to be holidaying from their female counter-halves.
Prior to that it didn't exist. (Well, there is one famous image of Al Capone smiling with a cigar from circa the Depression/Prohibition/Bootleg/Chicago era, but not at the camera, so it doesn't count).
Even Oscar Wilde, the funniest man who ever lived - as well as the saddest - couldn't so much as even as summon a smirk in his overwrought portraits of the late 19th century. No, smiling for and at the camera is a thing of the last 72 years. I can muster many explanations as to why it is a new phenomenon.
No one in her right mind would smile during the Great Depression. With decent meals scarce, irritable hypoglycemic males abounded. A hungry man's an angry man, as the saying goes.
The caretaking ladies in not being able to adequately feed their spouses, consequently, bore the brunt of masculine vocational anxiety and felt the wrath of his emotional, alcohol-addled froth.
It wasn't easy in those days: outfits were dowdy, uncles were rowdy and beans were a luxury. People were falling pregnant by ill-conceived means all over the place. Table manners must have been considered obsolescent owing to the widespread hypoglycemia. Naturally, the last thing one wanted to do was pay for a photograph, much less smile in one, unless, of course, you were Al Capone.
Going back a few years, the roaring, whoring 1920s saw people too drunk, doped-out and smoked-out to smile for a photograph. It was as simple as that. Heck, the only photographs from that era that exist are of those of gangsters, movie stars and writers living it up on the French Riviera, and the liquid lifestyle certainly abundantly agreed with that lot then.
Basically, the boredom and guilt of no wars coupled with vast economic prosperity drove the well-to-do to overdrink, overthink and avoid being photographed. I think we've just discovered the Great Depression was the comedown to the roaring '20s - an era of hangover finance, or lack thereof, as it were. Anyhow, Capone was the only one smiling in both eras, photographs aside.
Who the devil would smile during the Great War? I mean, the career-lifestyle options were nil and void. You either could be blown to bits or be Jack London, D H Lawrence or Katherine Mansfield, and those three weren't exactly cheery.
Mind you, a photograph back then was probably as exciting as a paddle board is now. (I think I've just answered my own question; no wonder no one smiled.) So a photograph would only serve to compound your worries, if you were a screwed up writer. Personally claiming to be and posing as thus, I should know.
Fast-forward a few decades, the 1950s were the 1950s, so people were far too bored to smile; the 1960s saw both tough and toothy as the in-looks with the surly Rolling Bones on one page and the over-smiling Beatles on the other (until Rob Dylan introduced the latter to cannabis whence they subsequently just looked zonked); the 1970s, with its immense beards, ensured only ladies smiled in pictures.
In short, there wasn't a social expectation to flash one's pearly whites in the aforementioned eras like there is now. Hell, the only ones getting away with not smiling these days are horses, dogs and plates of food. Even babies and, especially, toddlers are expected to have enthusiasm for the innumerable snaps they're subjected to nowadays.
People had it really good in bygone days.