|I asked the same question when I was younger. There isn't just one reason.|
Partly Tradition Early photographers who used film needing long exposures would not have been able to cope with a smile. What sitter can hold a smile without quivering cheek and lips. Relaxed facial muscles are far more likely to produce a good portrait. It became a tradition that people were discouraged from smiling; it was a safer option.
Perhaps Dentistry Some people in those days had bad or missing teeth and would not want to smile.
Certainly Fear A portrait was a very big occasion for the sitter. Our grandparents found the whole experience daunting and intimidating.The photographer had enough problems dealing with shaking knees and hands. This event only happened perhaps two or three times in a person's life. The evidence can be seen in the photograph. The reason the subject is leaning on an aspidistra stand and holding a book or flower, is to keep their hands still. They were terrified!
"In those days only lunatics smiled and grinned for no reason"
|Quite a few people explained to me that the whole concept of grinning for no reason would be alien to them.
Our forefathers would find the idea of smiling at a camera in the same way you felt when you left a message on a telephone answering machine for the very first time. You found it ridiculous speaking when you knew nobody was there listening.|
Your grandparents found it ridiculous to smile at a box when there was nothing funny happening.
Generally men take themselves too seriously to consider smiling.