Answer:
The United States desired the West because the West coast would open up the United States to Asian markets. Eastern ports connected the U.S with Europe. The West meant we could expand our trading power globally.
The population was rapidly growing in the East, Americans were running out of room, they had to move West.
Economic hardships (panics and depressions) in the East pushed Americans to the West.
The West had abundant, inexpensive, and often free, land. This not only attracted eastern Americans, but poor, landless Europeans as well.
The British had their eyes on the West as well. Prior to the Annexation of Texas, Britain had established a friendly relationship with the Republic of Texas. They were interested in trading with Texas and keeping the U.S from expanding West. In order to limit British influence in America, the United States decided to Annex Texas in 1845.
