I cannot overstate the importance of your answer to this question. The interviewer, ideally, should not have a doubt that you want to be in investment management for the long haul. You should come across as mature, steadfast in your decision and appear as though you have thoroughly researched all your options and come to the decision that IM is for you.

The answer to this question is very personal and if you can't come up with a sufficient answer pretty easily than, honestly, this job is probably not for you. I will say, however, that this is a great opportunity to let your interviewer know that your personality lends itself favorably to being successful in the capital markets. Humble people who are competitive, inquisitive/perpetually seeking to learn, and intellectually honest often do well in this business.