To me it's just the simple fact that there's nothing else I'd rather be doing with my life and it's always been that way. Yes, the crap you have to go through to get to the job at the end of the tunnel sometimes seems ridiculous, but I just know that in the end it'll all be worth it.
Assuming there is an end of course.
Plus, as one of my instructors once put it; I'd rather fly than work for a living.