I never considered careers seriously until 13 years, with the secondary school coming to an end for me, I begun to ask myself what I really wanted to do. Since my strong suits were biology, chemistry and physical education (lol :d) at school. I would say I was academically strong but certainly not very dedicated.
I always appreciated the role of using a scientific basis to improve the healthcare of animals, human beings coming later. I discovered by my general personality that I was someone suited to an ever evolving job that requiring lifelong learning. I felt I had the foundation of interpersonal skills required and that I would enjoy the diversity and broad career options medicine provided. I honestly didn't considered the down sides of pursuing a career in medicine, including long work hours, many exams and the stress of difficult patients, law suits etc. I felt confident that the positives far outweighed the negatives after considering these aspects.
I really appreciate reading and i'm an inquisitive person, in short i'm kind of a "geek"; I also really enjoy working with and understanding people. I chose medicine because it offered me the opportunity to integrate these interests and apply them in a very practical profession. From examining patients to phlebotomy to suturing, it's not just about memorising information, but about applied skills. I'm now in my second year and I find it hugely rewarding to put together all the pieces of the puzzle- the history elicited with good communication skills, the knowledge of human biology and the findings from examination and investigations to suggest a diagnosis and be told it was the right one! I only found out that Medicine was my thing after passing to the second year. And the decision to stay a student for 7 years when my friends will be all in a gainful employment in 4 years wasn't an easy one. But it was the right one for me.
TL; DR: Looking back at past years, I feel the only way to be sure on what career you should follow is to get as much experience and insight into the profession as possible. Of course this applies whether you are 17 and at school, or if you are 35 and working in industry. Even then, you may leave yourself with a small leap of faith, but you are much less likely to be disappointed in the outcome than otherwise.