There were multiple reasons for westward expansion, including everything from ideological to practical motivations. Many Americans, particularly by the 19th century, believed it was an intrinsic right to claim land in the West. Others moved for financial reasons or to affect the balance of free and slave states.
One of the most practical reasons for expansion to the West was overcrowding in already populated regions. By 1840, for example, 40 percent of the national population was cramped in the trans-Appalachian west. With the Jefferson presidency, the United States acquired vast quantities of new land through the Louisiana Purchase. Jefferson himself, along with many others, felt that moving to and acquiring land in the West was an inherent part of the rights and liberties granted through the New Republic.
This mentality gained even greater ideological sophistication with the notion of manifest destiny, a theory that achieved great popularity by the 1840s. According to manifest destiny, Americans were entitled to land in the west by God-given right, an entitlement that was completely harmonious with the American experiment in liberty.
However, with the important slave debate compromises of the 19th century, it also became increasingly clear that new territories brought into the union must be equally divided between slave and free interests in order for peace to prevail. On the other hand, each camp saw acquiring new territories as vital to the survival of a way of life, often to the detriment of others. In most cases, white settlers and federal authorities deemed the rights and entitlements of native peoples to coveted lands insignificant.