I would say the most important reason is the lack of competition. Native tribes had been mostly conquered, and the Spanish, French and British empires were distant and preoccupied over time, so much so that it wasn't that difficult for the American empire to emerge as the dominant force on the continent.
April 30th, 1803 set the course for what America would call 'Manifest Destiny'. Although there were people who had already crossed the mighty Mississippi, in 1803 with the stroke of a pen and payment to Napoleon,Thomas Jefferson laid out the United States' destiny. The Louisana Purchase is still considered one of the greatest land deals ever neogotiated. The phrase 'from sea to shining sea' gave credence to who and what the United States' intentions would ultimately become. Jefferson would send Louis and Clark out to find out what he had purchased, and as the messages arrived countless people ventured out into the 'promised land'.
To toss in a few more:
* The spread of European diseases in native populations, which decimated them and weakened their ability to resist westward expansion
* France’s decision to abandon its North American colony, resulting in the Louisiana Purchase
* Land acquisitions resulting from annexations, treaties to end wars, and various other agreements with foreign countries (duh!)
* A tremendous wealth of natural resources and navigable rivers
* The advent of the Oregon Trail (and others) and the transcontinental railroad
* financial panics (1819, 1857, and 1873) that led to depressions, resulting in extensive urban unemployment, rural poverty, and an increase in westward migration in search of a better life
* and lumping together some of the arguably unsavory aspects: the federal government reneging on land treaties with Native American nations, slack enforcement of homesteading laws, and the widespread acceptance of the ideology of manifest destiny
I think you can probably trace the growth of America almost strictly to population. Once America was established as a great place to immigrate if you didn't like your own home, it was impossible for everyone to live comfortably without expanding west. This is why various nationalities tended to group together in new places; it provided the familiar feeling of "home" life with the freedom of being in a new land. As more people of that nationality landed at Ellis Island, they were directed, either formally or informally, toward those concentrations of their own people. Then newer groups kept getting shoved farther west. Eventually, the discovery of gold on the west coast completed the transition.
Some of the key reasons for the expansion of the United States include population growth, the increasing expense of land along the eastern seaboard, and the insatiable desire of Americans to own land.