In the 1950s it was expected that you would get a job after college and probably stick with it for the rest of your career. In modern times, having a single job or even only two or three jobs is very unusual. There are several reasons for this. First technology is driving a number of changes and many jobs that are around today won't be around in the same form even ten years from now. Second we have become a much more mobile society so it isn't at all strange for people to several times in their lifetime to far flung towns. Third businesses approach to human resources has changed dramatically. Companies expect to be able to grow and contract with the economy. When they contract people lose jobs. When they grow, there are often great opportunities and monetary rewards offered to people for leaving their existing job.
If you expect to change jobs several times you can prepare.