the concept of smiling

I was doing research for my essay and I found a picture of some men at a factory in an office just hanging about. It struck me that not a single one of them was smiling. Then I realized that a lot of people in old pictures don't smile. So, when did the concept of smiling in a picture come to life? 

Because it's a pretty weird concept. When did we start thinking, okay, everybody, we're going to document this moment possibly forever so look as happy as possible. Not, so just keep doing whatever you're doing in the moment because if you stop to smile and stare at the camera, it's not really the moment anymore, is it? That's why I like candid photographs so much more than stop-and-smile ones; the candid ones actually portray the moment as it is, for the most part. I mean, there are some pictures of the stop and smile variety that do too. But I guess I just don't see why a whole line of people has to get together and smile as brightly as possible or else it doesn't look good. 

Why do we smile in pictures? When did it become not okay not to smile? Is it an obsession with being happy that we feel necessary to show the rest of the world? Why are we obsessed with being happy? 

I remember a girl from high school who I would see get her picture taken on several occasions -- the stop-and-smile pictures. Each time, she would smile as the camera flashed and then after the picture was taken, she would put on a much more genuine smile that looked so much better on her. The one in the picture was fake. So, what is even the point?

Also, I'm quickly coming to the realization that my essay is going to be the exact replica of the example of a bad essay our lecturer gave us in class. Woohoo.