You would think that Finland's unemployment rate (10%+) would influence its ranking, but that's not the case at all.
Nordic countries have better safety nets.
I haven't travelled there but I grew up in Poland and still visit. US feels very capitalistic to me. I feel the pace is slower in Poland. In US I feel the need to produce. Might be just me.
Well, that's just inherent in the question which asks someone to imagine the best possible life vs. the worst possible life. In a society with lots of room to grow you aren't at the higher rungs. In a society with no progress possible you're at the top easily.
As it's selfreporting and it's more about expectations than actual happiness a finnish dude only needs to think that life is just incredible compared to what he sees at the other side of the border to selfreport a 10 in happiness