My main reason was because I saw critical illness within my family at a young age and felt pretty much helpless at the time. I saw the work of doctors and other healthcare professionals who more or less saved the lives of some of the most important people in my life. Therefore, I couldn't think of a more valuable or fulfilling or personally meaningful career for me. Not to mention the fact that I'm pretty fascinated by the human body, the way it works and the ever-increasing knowledge about how to keep its systems in balance.
That... or the fact I used to run around as a semi-naked 5 year old playing doctor?