Causes of westward expansion included missionary purposes, a need for adventure, desire to be away from crowded cities and hope for gold on the west coast. The majority of westward expansion happened as a result of manifest destiny or the belief that the colonists were superior and needed to spread their superiority as far as possible.
People who wanted to spread their superiority went looking for Native Americans in parts of the country that had not yet been established. They wanted to teach them about the way they lived and force them to live the way that they did. Many of these people saw themselves as missionaries and believed they were working toward a greater good.
Some people also chose to move out west because they were tired of the big cities and the poverty that was enveloping the East Coast. They wanted to get away from the industrialization and the expansion that was occurring. Other people moved westward because they dreamed of striking it rich. They had heard tales of people who went to California and found all of the gold they could ever imagine. This part of the expansion was called the Gold Rush and made up the majority of settlers in California.