The five factors that led to westward expansion was the increase in population, new opportunities (economic), social, political and slavery.
The increase in population made the current territories crowded and opportunities did not seem as abundant, so individuals sought out new territories.
In expanding to new territories would bring about the potential of new economic opportunities which equates to labor, wealth and status. The social and politic ideas allowed individuals more of a choice rather than by majority or force.
Lastly, slavery impacted economic opportunities and there was a lot of controversy if slavery should continue in the western territories.
I believe the two factors that were most significant in getting Americans to migrate west are population increase the (economic) new opportunities.
The start of the expansion began with the Louisiana Purchase in 1803, President Thomas Jefferson negotiated the land for purchase which eventually turned into 14 states.
Americans eventually negotiated further west to acquire Oregon and California (Gold Rush) among many others.
Gold, silver and the Homestead Act are some of the many factors why individuals decided to move rest (land and opportunity).