Frederick Jackson promoted the "Frontier Thesis," arguing that the American culture and democracy came about as a result of the (moving) American frontier.
The first settlers in America from Europe (in the 17th century) carried with them the European culture (e.g. established churches, aristocracies, standing armies, unequal land distribution and an intrusive government). Eventually, the descendants of these pioneers moved further inland of America (i.e. towards the west).
They had to abandon certain European practices (e.g. of hierarchy, and certain established and rigid ways) that were not beneficial to them as they adapted to the new political, physical, and economic environment -- marked by savagery and the wilderness. These factors drove the succeeding generations to develop the American culture-- more democratic, more individualistic, more violent, and less artistic and less scientific. Generally, the further inland that the people went and lived, the more American they became.