Lots of people will tell you about how old cameras take a long time to develope the photo and you have to sit still for a long time so people could not smile. This is not true since the vast majority of Daguerrotypes were taken after 1845 and the exposure time was shortened to seconds. According to the book The Daguerreotype in America, photography was much cheaper than hiring a painter, but early photography was still expensive enough that people would consider it a serious occasion akin to having their portrait painted.

After Kodak made camera portable and affordable after the 1880's, people realized they don't have to look like stately portraits in photos anymore.

Think about how many bad posed pictures you've taken in your life. Smiling very often comes out, well, bad. The last thing you want is to take that one picture you might ever have taken of you in your entire life and look completely goofy. Once you reach the 1920s cameras are much more everyday and their processing allows for multiple images to be taken without massive cost. Most of my relatives photographs between 1880 and 1920 they look pleasant, but aren't grinning.