Why do we smile in photos?

Ever noticed how no matter when or where a photograph was taken in the 19th century, it was incredibly rare to come across a single smile. Surely they can't have been that gloomy back in the day (must've been all that evil smallpox and what not) , right?

Except for that smiling guy in the other post.

But hey, let's figure out WHY our long-gone 19th century colleagues seem to be so cranky. This is by no means a complete list but rather the logical assumptions.

(Source)

1. Primitive cameras with long exposure times 

Whereas we live in an age where we capture that split fraction of a second in time and send it to our colleagues on Snapchat or Twitter (or God-forbid MySpace), our deceased colleagues weren't as fortunate. Cameras were sturdy things, they had incredibly long exposure times often requiring people to stay rather still for several minutes or else the photograph would blur out. In case you haven't tried it, smiling for several minutes whilst remaining still (like you would if you won a lottery) is incredibly hard to pull off unless you're a masochist and love some serious muscle aches. 
  

2. They were expensive and you don't want to look stupid when applying for that one job

 Who knew our rotting colleagues had so much in common with us?  Getting the opportunity to have your picture taken was serious business. These photographs were basically a permanent profile picture that was carried around their whole life. As a result, it was taken very seriously and people strived to look as passive-aggressive as humanely possible.  
Mark Twain's attitude basically sums it up;

A photograph is a most important document, and there is nothing more damning to go down to posterity than a silly, foolish smile caught and fixed forever. 

It's also worth noting that the influence the art world of portraits had on the early use of photography.  Nicholas Jeeves commented, "Smiling in paintings was only seen in the 17th century in Europe, it was a well-established fact that the only people who smiled broadly, in life and in art, were the poor, the lewd, the drunk, the innocent, and the entertainment."

3. People had terrible dental hygiene

This really doesn't need any further elaboration. 

But why do we smile?

We're silly creatures, who knows why?