I'm sure that's part of it. Another hypothesis I like is that the US was populated to a significant extent by people who, when faced with problems at home, didn

I'm sure that's part of it. Another hypothesis I like is that the US was populated to a significant extent by people who, when faced with problems at home, didn't stay and fight to fix them but instead preferred to leave. In addition to migration patterns that selected implicitly for such folk, this happened internally to the US as part of the movement to colonize ever further west. Now, there hasn't been anywhere else left to go for a while, but society hasn't really internalised this yet.