*Sorry for the necro-bump, but I wanted to add to this*
I decided to pursue a career as a physician more through a means of deduction than anything else, and I don't think that's any less noble than someone who spontaneously knew they wanted to do medicine or gravitated straight to it. Throughout college I was always interested in the sciences, but more so on its application. After college I started looking at graduate school programs and what I realized is that I wasn't interesting in any application of those sciences, but their application towards the human body, us, our specie, and our civilization. At the time I was considering graduate school programs in medical chemistry, environmental chemistry, toxicology, epidemiology, etc and not just organic chemistry or zoology.
However, I realized that going that route would close the door to the experience that I believe I find most fulfilling: directly affecting change to those around me and caring for their health. I've had jobs that were rather asocial and it was not an experience I enjoyed. I want to be able to exchange tears, laughs, and happiness with others to the same extent that physicians do. Medicine allows me to apply the sciences to a degree that not many other sciences allow for. It has a strong humanitarian component, as it pertains to justice, but also morality. It involves so many aspects of our society.
This is kind of a dorky answer I think, but it is my own personal story of how I arrived at medicine. I was gritting my teeth trying to answer this question for interviews, and I finally came up with something that wasn't generic necessarily or a regurgitation of other comments. I'm not sure I will be able to vocalize this at interviews, but maybe some of this will get across. I like to be succinct.