mmm interesting... for whats its worth i think that its a totally personal thing. Some do it for a career, like any job but the majority do it for the love, passion and a deep down desire that no other job will ever do! How can you explain why you would put yourself through the financial and emotional upheavals and sacrifices to achieve a dream that you didn't love or felt you were destined for?
For me, I come from a aviation background and I was fortunate as a 3 yr old to have my pic taken in the cockpit of a Red Arrow.....from then on i can't remember wanting to do anything else. I tried and even gave up flying for a couple of years for other reasons but became exteremely miserable and despondant....so for me nothing else will do. Flying has taken over my life and I couldn't be happier. Now as a nearly qualified FI, I can share this v.cool life with everyone else!!