Answer:
the discovery of gold - contributed to the westward expansion, especially to California, during the California gold rush of 1849. Hundreds of thousands of immigrants, both from the U.S. and abroad, arrived in California in this period, lured by the prospect of finding untapped wealth, and even after the gold ran out, many of the stayed, contributing to the ensuing population of the rest of California and the Western U.S.
the quest for land, wealth, and adventure - most of the first settlers in the Western U.S. were adventurers, explorers, hunters, fur traders, and mountain men, who entered this new and unexplored territory in the search for land, wealth and adventure.
increased immigration - during the second half of the nineteenth century, immigration to the United States increased dramatically, and many of them settled in the Western part of the country, where they could purchase cheaper land.
the Homestead Act - this act provided for land grants to anyone who had not born arms against the United States. The person granted the land had to cultivate it and improve it in exchange. This benefited hundreds of thousands of farmers, who mostly settled west of the Mississippi River.
