In early photos (or daguerreotypes) people invariably have a serious or even solemn look on their face, which I believe is in large part to them considering them another form of portraiture. But somewhere along the line people started smiling. When did that start happening and why?
Not discouraging more information here, but FYI, there have been a few threads on this that you may find interesting:
When Did People Start Smiling in Photographs? featuring /u/Axon350
When did smiling in photographs become common? - more from /u/Axon350
When showing teeth (in art, photograph and portraits) became culturally acceptable? And why it wasn't before? - /u/GeeJo gives a quick survey of smiling in art
These threads are all archived, so if you have follow-up questions for any of the users, just ask here and tag their username