What was the major incentive in the settling of the west?

During the time when North America was still being settled by European colonists, it was possible for settlers to go west and to claim land. The east coast was the first part of North America to be colonized by Europeans (being closest to Europe), so that land was the first to be claimed. After all the land was claimed in the east, there remained land in the west, still available to be claimed (given that the prior claims of native Americans were generally ignored).