I don't know if this is a good answer, but if you asked me right now why I want to become a doctor, the God-honest truth is that I just can't see myself becoming anything else.
I'm curious, I'm friendly, I'm personable, I'm not squeamish in the least, I have a very long attention span, I'm cool under fire and I enjoy solving problems. I've been interested in biology forever, and outside the classroom my chief hobby in life is to figure out how to get the most from the human body via diet, exercise and working the mind. I'm good with my hands, which I demonstrate by the music I play, and I have a good memory - not amazing, certainly not the best, but good enough. I like explaining things to people, and when I work with groups (as in a hospital setting), I like to make sure that I am a good hub for information, and that I don't let any miscommunications go unnoticed, or any relevant people left in the dark. I don't seem to need much sleep and I am pretty good with time management. I'm a doer, a thinker and a tinkerer. I'm always working on projects, be they musical or physical (automotive work/alcohol brewing/leather working/house remodeling). I'm pretty sure that if I became a physician, getting to the bottom of human problems and figuring out how best to fix them, planning out surgeries and pondering medication conflicts and complex diagnoses would just be something newer and more substantial to finally sate my natural tendency to want to tinker and fix things.
I didn't choose medicine, medicine chose me. If I was born to do anything else with any satisfaction, I certainly haven't found it yet.