If you’re Finnish, I might say you don’t often smile but I bet you know how to neatly drift a rally car through a forest at considerable speed.
Why is some generalisation good? And others bad?
Finns generally demand a larger personal space and thus may appear as "cold" to someone used to being in closer proximity, but it is what it is.
Believe me, I'd love to be able to act less reserved around new people, but I find it hideously difficult, and if I don't feel outgoing, I can't force myself to act that way because it causes me intense discomfort.
As for smiling, it might just be that for Finns a "neutral" face is perfectly polite and a stranger's smile elicits a stronger reaction (either positive or negative depending on context) while people from the US might expect a smile as the default state and thus react less strongly to it.
While I might really be enjoying a good walk in the sun for example, I can't really say that it makes me want to smile at every random passer-by.