it's 100% cultural. For example native americans rarely smiled, even to this day native peruvians for exampe don't smile a lot. Same for chinese and japanese, but they've been catching up with the fake ass smiles since we've been in the television era.
Europeans and earlier americans don't spend their time with a fake ass smile on their face like modern americans do, but as I said tv/cinema culture tends to homogenize the bullshit.