I reckon it’s probably to do with internally trying to downregulate the smile so it doesn’t look really weird or crazy or fake or stupid and you just overcompensate. You don’t know what you look like while you’re doing it and that information void gives rise to some self consciousness and pre-emptive embarrassment. Natural smiles don’t require you to think about how to do it at all so most people don’t really know how to smile in a socially acceptable way on demand with a few seconds notice. Some people are better at it than others, maybe they have a better intuition, maybe they have a better awareness of what muscle movements correspond to what changes on their face and also a really good grasp of which tiny subtleties lead to a photogenic smile or the grimace of a maniacal murderer. Actors are probably pretty good at it either through intuition or just a lot of practice. I should imagine you could train yourself to be better at it, but it’d feel weird and vain to spend your time doing that so a lot of us just make weirdly flat or stern faces in photos.

If you can actually just enjoy the moment so it makes you smile as a result it’d probably get better results but that idea leads to its whole own self reflexive internal monologue trying to concentrate and force yourself to be happy that probably results in a frown while you summon that concentration.