I have to agree with Atsai. This is a VERY important question both for the program director and for you personally. If you haven't thought long and hard about why you're pursuing training in IM then you owe it to yourself to do so before you apply and match!
Also, think about it from a program director's perspective. Why would you want to hire someone who isn't really sure they want to do the job? As much as applicants get annoyed by the "tell me about yourself" interview questions, they're really great ways to gain valuable insights about an individual. And as Atasi said, this shouldn't be about what you think they want to hear. Rather, it should be about what YOU think and feel, in your gut, in your heart, about why IM is right for you. Programs are trying to see if you're a good fit with their departmental culture, and if you misrepresent yourself you're doing both yourself and that program a potentially disastrous disservice. I don't think it's a good idea to be anyone but yourself in this process, lest you end up somewhere that really isn't right for you.