Early photographs had very long exposure times, in fact the first photograph of a person, Daguerre’s photograph of the Boulevard du Temple taken in 1838, had an exposure time of about 7 minutes and the man was only captured because he was standing still to have his shoes shined. It would have been difficult to hold your face perfectly still for longer than five minutes, let alone while smiling. In fact, there were often devices like neck cradles used to hold people still, as any movement could cause blurring. The social norms around photography were also different at the time, it was an expensive occasion and would have been treated quite formally. It was so expensive in fact, that people in the late 19th to early 20th century would save up for photographs to be taken of someone after their death to remember them by. Death photographs were a continuation of the traditions of death masks and physical mementos such as locks of hair. So photographs were not only limited by the technology, but also taken quite seriously which explains the expressions. As for smiling in photographs, the first point and shoot camera was released by Kodak in 1888, however it was still very expensive and mostly a novelty for wealthy people, and the film still had to be sent in for processing. The first "instant camera" that printed photos automatically was released by Polaroid in 1948. This unit was more accessible and sold in department stores, and the fact that the photos were processed and printed automatically gave them a "candid" air. Essentially you could take photos and no one would have to see them to process them, you could take silly photos, racy photos, whatever you wanted. Candids became a popular form of photography, and this trend reached a peak with the development of disposable cameras. Although they were first released in France in 1966, they didn't really take off until FujiFilm released the Utsurun-Desu in 1986. Disposable cameras were perfect for traveling and vacations since they were inexpensive, so you didn't have to worry about loss or damage. Kodak leaned into this concept hard in their marketing, renaming their disposable camera as the "Funsaver" in 1989. So essentially, as photography became more ubiquitous and less formal, the norms surrounding it changed. With the emergence of candid photography in the 1950's, it became a popular style to try and capture events and happy memories, which meant that people were typically smiling in the photos.