Hmmm... In a sense, I am an outsider to "American Culture", although I live in it. I would say that it's not so much a "HAPPY HAPPY HAPPY" thing as much as Americans don't like hearing that everything isn't going well for someone- maybe there is a cultural fear that "Maybe I won't have it so easy too!"

It's not that they want people to always be happy- it's that they don't want to be reminded that life isn't always easy or fair (although I would argue that is true because PEOPLE make it so.)

And then there is the stereotype: you do right and things work, you don't do right and things don't "work"- in other words, if you sin you get punished in your life and if you're "right with God" you are rewarded. When people are having a hard time of life and they're not overtly "sinning", that flies in the face of that stereotype- and American thinking and culture IS largely driven by stereotypes (and usually false ones).