I have a theory, it relates to latitude. The farther north a country is the more people may have to depend on their neighbours/community for survival. I'm no anthropologist, but if the winter is damn sure to kill people I think, you're likely to play nice with each other in the hope that if something bad is happening(bad harvest, dead adult provider, ect.) they help you out somehow. This is entirely made up bullshit however.
:edit: Also meant to say, I based that off stereotypes. I really haven't run into the "rude American".