A combination of:
1. Geography. A abundance of land and resources along with relative isolation from other nations that kept the US out of major wars.
2. Small government. For most of the United States' history, It has enjoyed low taxes and small government that didn't interfere in people's lives unduly.
3. A Northern European culture. That is, a combination of mainly British and German culture with some Scandinavian bits tossed in. This was rooted in the liberal thinking of the 17th and 18th Centuries with people like John Locke.
4. Self-reliance and the need to tame what was mostly an untouched continent.
What's doing the US in is:
1. Big government. Between the industrial-military complex, social-welfare state, and general growth of bureaucracy and taxes, government is strangling the life out of the US.
2. A cultural shift to the Left and away from traditional liberal values. Toss in a marked rise in cultural intolerance that marks most of the world today, all of it coming from the Left. It doesn't help that the Left is pushing to shift the US to Middle Eastern, S. American, and African values all of which SUCK. This is done in the name of "inclusiveness."
3. Cultural and moral decay. Like the late Roman empire, the US suffers from its own success. In a word, the US has grown lazy.
4. Lack of focus or strategic goals.
