Continuing from the above post, another way in which the American psyche was shaped by the Westward Expansion was in the development of the attitude that it was always possible to move to a new location and begin again. Americans, for many years and generations, didn't have to think about staying in one place and dealing with the hardships and complications that might have come to that place. They could move and start over in the hopes of finding a better situation. This started with the thinking of the immigrants who came to the "New World" in the first place - the ease with which Americans move from one place of residence to another, the lack of reluctance to split up family units in order to find an advantage in another place, is quite unusual in much of the world.
A major theory is that westward expansion created the American character. It is said (this is a theory connected to a man named Frederick Jackson Turner) that the expansion allowed Americans to be individualistic, rugged, and tough because they had to be all of those things to deal with the rigors of "taming" the West.
One way that Westward Expansion changed America was that it brought the issue of slavery to the forefront of political discussions. As new states were added the fragile balance of power between slave and non-slave states had to be considered and compromised. One could easily argue that westward expansion started a chain of events that brought about the Civil War.