I had a real hard time articulating the answer to this question too when I began interviewing -- I had this horrible, typical feeling that I'd sound generic, or stereotypical, or redundant, or fake... etc.
So I said f*** it, I went honest, and yes, this is what EVERYONE tells you -- "just be yourself" But I did use the power of creativity to bolster some of my more boring arguments. I started off by talking about how I wasn't one of these kids who's always wanted to be a physician throughout their life (no offense to those people!) I just kinda always worked hard in school, living up to the hard work my parents had done before me, and earning their respect by emulating them in all their good qualities. By high school, I still wasn't too sure what I wanted to do, my parents of course always said "medicine" but I was only patronizing them by saying "yeah, I'll try to be a doc." So, by college, I chose biochem. as an undergrad major. I didn't enjoy this too much, but I enjoyed the challenge and did very well in my academics. Everyone kept telling me how hard it is to get into med. school, and I honestly just didn't think it was gunna work out for me (didn't think i had what it takes). But I kept at it -- kept the dream alive. I volunteered my junior year at a rehab hospital, gave up friends and family for a month, and put in near-40 hour weeks. Loved the experience and the gratification of helping the sick and disabled -- and even some of the dying. It gave me hope that I was at least .. AT LEAST.. pursuing the right career for me. Whether I'd get in or not was another story -- I was JUST as unsure as I was in high school. After pushing through the MCATS the first time -- getting a 26 (had mono diagnosed on me the day AFTER the test... no wonder I was sleeping through biological sciences!) .. I pushed hard -- depressed as ever, but completely motivated to TRY. Never gave up that ability to TRY. F*** what everyone says, I told myself, you'll do it. And six interviews later, I received my first acceptance. Then two more. Now, looking back, I think, what a cool experience, and this.... this is such a great answer to that age-old question:
So why do you want to be a doctor?