The reason people didn't tend to smile in early photo portraits stems from the painting tradition: In paintings, since the model has to hold exactly the same pose for hours on end, after a while a smile turns into a hideous grimace. They tried to keep a relatively neutral expression because it was more comfortable during long modeling sessions. When photography was getting started, the earliest photos took up to several minutes to produce so there was a similar strain involved with maintaining an active expression. Even when photographic technology improved and photos became faster to produce, the old painting traditions of modeling held fast for a while, especially in formal portraits. When amateur snapshots started coming into fashion at the end of the 19th-century though, photos were fast enough and the situations were informal enough that we see plenty of smiles and vivid expressions.
Here's an article about how technology influences people's perceptions: http://www.digitalvictorianist.com/2011/12/smiling-victorians/_
Some photographic collections of smiling Victorians:
"18 Photos of Smiling Victorians": http://www.buzzfeed.com/lukelewis/18-photos-of-victorians-smiling#.ckPWL3QRa
"The Smiling Victorian" A Flickr collection of over 2,000 pictures of happy Victorians (and a few Edwardians):
And here are some of my favorite photos of smiling Victorians from our own collection:
Small child: "Mama? When can we go to the candy store, Mama?"
Mother: "Soon, dear. Smile, now!"
Small child: "This place is boring, and that man just keeps hiding under that curtain attached to that funny box."
Mother: "He's a photographer, and that's his camera, dear. Please smile, for Mama's sake."
Small child: "When can we go to the candy store?"
Mother: "After you smile, dear. Please smile, now."
Small child: "I want candy..."
Mother: "Just smile, dear."