In the financial services field, you should have strong personal motivation to succeed in your work and your interviewer is asking this question to see what it is that make you tick while on the job. While being open and honest in how you stay motivated on the job, make sure that your answer doesn't include collecting a paycheck every week or two. While money is certainly a motivator in this field, make sure that you use it in an appropriate context if you decide to talk about the financial piece of this business as being a motivator. In the end, your interviewer is looking to get a sense of how he or she will be able to keep you hungry for more if you are hired for this role.
