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It remains a commonly held belief that for hundreds of years people didn’t smile in pictures because their teeth were generally awful. This is not really true – bad teeth were so common that this was not seen as necessarily taking away from someone’s attractiveness. Lord Palmerston, Queen Victoria’s whig prime minister, was often described as being devastatingly good-looking, and having a ‘strikingly handsome face and figure’ despite the fact that he had a number of prominent teeth missing as a result of hunting accidents. It was only in later life, when he acquired a set of flapping false teeth, that his image was compromised. His fear of them falling out when he spoke led to a stop-start delivery of his speeches, causing Disraeli to openly poke fun at him in parliament.

Nonetheless, both painters and sitters did have a number of good reasons for being disinclined to encourage the smile. The primary reason is as obvious as it is overlooked: it is hard to do. In the few examples we have of broad smiles in formal portraiture, the effect is often not particularly pleasing, and this is something we can easily experience today. When a camera is produced and we are asked to smile, we perform gamely. But should the process take too long, it takes only a fraction of a moment for our smiles to turn into uncomfortable grimaces. What was voluntary a moment ago immediately becomes intolerable. A smile is like a blush – it is a response, not an expression per se, and so it can neither be easily maintained nor easily recorded.

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– http://publicdomainreview.org/2013/09/18/the-serious-and-the-smirk-the-smile-in-portraiture/