For what reasons might you change jobs?how often would you expect to do so in your life?isit a sign ofsuccess?

Because the workplace is poisonous, either literally or metaphorically. Because your boss is a jerk. Because you've got a better offer. Because you believe you can do better. Because you want to change what you do.

Because you need a different situation, to enable you to go to school, or have a child, or move to another area, or for medical reasons.

It really has nothing to do with success per se. Some successful people have bounced around from job to job all their lives - others have been successful sticking with one job.