OP, you probably shouldn't be discouraged by someone asking you why you want to be a doctor. It's a quite sensible question to ask, considering the arduousness of the path you'll take to get there. You're adviser just wants you to think about what might be one of the most important questions you should ask yourself before taking this route. If you have trouble finding an answer to it, I'd spend some time trying to find one.
Your adviser is trying to weed out an enormous and miserable portion of the initial premed population that doesn't make it through: kids whose motivations to be doctors lie outside themselves. These are kids whose parents are both doctors; or who want to be able to say "I'm a doctor" for the status, without actually considering what it entails; or who have watched too much Grey's, and imagine a world in which doctors are all as physically attractive as movie stars and as sexually active as the emperor's favorite concubine.
That said, I definitely agree that advisers / counselors can be awful people. I took a double-digit number of AP tests in high school and did well, got a near perfect score on my SATs, was 3rd out of my class of 750 in high school, and began taking college courses while I was still a high schooler. My college adviser told me that my state school was without doubt the best I could do, so I needn't bother appling elsewhere - unless I wanted to go to community college for a year or two, acclimate myself, and then apply to transfer to my state school. She also told my brother this, who was also academically impressive, on top of being an All-American athlete.
I'm at an ivy now, and am doing great here. I'm the only one from my class who went ivy my year, and there were many other intelligent and talented students. I'm convinced that the nay-saying of our advisers contributed to this.
