It’s just a cultural norm in certain parts of the world, to be supposed to force-smile on photographs. Jst smile naturally, be as beautiful as a Mona Lisa.

… and by the way, not all cultures recognise a friendly face by how the mouth is grinning. The easterners tend to look at the eyes. That’s why the old Japanese smileys (in ASCII or unicode characters before emojis were introduced) focus on the eyes, rather than only on the mouth. kaomoji (wikipedia) – At least that’s what i learned once. (^_^)