The causes of westward expansion during this time were much like the causes of westward expansion at other times. There was the "pull" factor of cheap land and the "push" factor that caused immigration from Europe and a shortage of good jobs in the East.
Most of the reason for westward expansion was economic. People wanted to be able to carve out a better life for themselves. As the west was being "opened" there was a great deal of relatively cheap land to be had. This appeared to many to be a good way to improve their economic situation.
At the same time, there were many immigrants coming over from Europe as they fled economic and political hardship. These negative conditions in Europe (as well as some in the East of the US) pushed people to "go West."