The expression of most Victorian models isn't so much "serious" as it is relaxed. 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.
Some of my favorite photos of smiling Victorians from our own collection:
A friend who has children herself suggested it looks like the mother is using her right hand to tickle the child so she will laugh for the picture.
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."
Photo of children
Photos of couples and families
Photos of groups
Photos of maids
Photos of men
Photos of nurses
Photos of various women
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):