What prompted westward expansion in the 1840s?

The philosophical impetus for Western expansion had been provided long before the 1840s by Thomas Jefferson. His somewhat idealized conception of the republic was based upon a nation of independent, self-reliant farmers, both large and small. Jefferson shared the almost universal prejudice of his class that rural life was superior to life in the cities. Ownership of land gave people a stake in society; their connection to the soil provided stability and a sense of responsibility which, in turn, provided a large pool of citizens from which the nation's governing class could be drawn.
Expansionism is also prefigured in Madison's discussion of the dangers of factions in Federalist No. 10. Madison argued that the only way to minimize these dangers was to deal with their possible effects; one of the methods to achieve this was to greatly expand the size of the republic. This way, he believed, it would be harder for a majority of citizens to form themselves into a factional tyranny, as they would need to persuade more people of the justice of their cause.
Prior to the 1840s, Western expansion was not only a matter of theory, it was a reality for millions, both white settlers and Native Americans alike. However, in this particular decade, the process accelerated rapidly. One of the reasons for this was economic. In the eastern states, industrialization was growing apace, changing the very foundations of society, along with the nature of the economy. More and more people found themselves working for the first time in factories and large-scale manufacturing plants that spearheaded the new industrial revolution. 
Inevitably, with such rapid industrialization came a number of serious social problems, such as crippling poverty, overcrowding, and disease. As conditions in the cities deteriorated, many looked to the wide open spaces west of the Mississippi. Many hoped to be able to live out the dream of Jefferson's rural idyll. The opportunity to lead a free, self-reliant life on one's very own land proved irresistible to millions.
However, a number of other factors prompted western expansion, not just those related to the acquisition of land. The Western territories' vast, largely unexploited mineral wealth held out the prospect of untold riches to those who previously could have only dreamed of getting rich. The greatest example of this insatiable lust for gold is the Gold Rush of 1848, which led to an enormous growth in California's population and the establishment of its eventual statehood.
The hunger for religious freedom must also be taken into account. In 1848, facing widespread persecution in the Midwest, the Mormon Church, under the leadership of Brigham Young, took part in an exodus to the Great Salt Lake Valley in order to establish their godly kingdom on earth.
In examining all these factors, we can detect a common element. All those who headed west, for whatever reason, were attempting to realize for themselves the promise of America. Land ownership, wealth, self-reliance, and religious freedom had long been integral components of the American psyche, taking on the character of almost a civic religion. When the journalist John O'Sullivan described Western expansion as the nation's "manifest destiny," he was simply coining a pithy expression that articulated how many Americans already felt. What the Founding Fathers had achieved back east, a new breed of pioneers would achieve in the wide open spaces of the West. 
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