While there are many explanations to this, and the question itself may sound rather odd, why do people smile in pictures? If you're genuinely having a good time or are happy, then great! Glad you're having fun. However, in most cases that's not the true story. You're arguing with family/friends then someone pulls out the camera and suddenly arms go over shoulders, you muster up an artificial smile, and as soon as the picture has been taken people immediately go back to their unhappy mood. Is it to preserve the memory as a "happy" one? Is it to show your family/friends of how much of a good time you had, even though it was the complete oposite? Wouldn't you be lying to yourself if you smiled in a picture even if you weren't happy, just so the memory would seem like a good one when you look back upon in years later? A false memory built on a lie?
We've all grown up, accustomed to "smiling" when someone wants to take our picture, in the same way that most of us respond with "Good" when someone asks the very generic "How are you?", even though that isn't necessarily how we are or how we feel. I remember watching a documentary of a charity organization bringing cameras, PCs, and other technology to various villages to Africa, in order to promote education and such. After figuring how all the technology worked, the villagers, when taking pictures of themselves, never smiled, because they had no reason to muster up an artificial smile. They had never been exposed to eurocentric/western "customs", and quite frankly, what they did made sense to me.
Enough ranting from me, I made this thread to see what all of you think.