I train because it has become part of the warp and weft of my life. It is my 'way', what I do. It is what I understand as a 'do' (Japanese 'doh', not American English 'doo').
Self-defense is part of it, physical fitness (or more honestly, physical activity) is part of it, but the real reason I train now is because I enjoy everything about it. Mostly, though, it's internal now. It's what it does for my spirit, rather than what it does for my fists. Yes, I still hit like a train and kick like a mule, but if that's all I wanted to do, I can do that now.
It is the settling of my weight, locking to the floor. It is the spring tension as I receive a blow, or the uncoiling power as I strike. It is the speed and precision and control which I find within myself. It is my breathing, coordinated with my movement. It is the moments of clarity as I discover some new nuance, some new minute adjustment to my posture; or my attitude, which I had previously been unable to perceive. It is the manner in which I live my life outside the dojo and inside, and how little by little, my outside life comes to represent the lessons I have learned inside the dojo. Self-restraint, courtesy, kindness, resolve, steadfastness, determination, even humor.
Everything I ever wanted for myself as I looked for my purpose in life, I have found within the walls of a small nondescript dojo, surrounded by friends who were once strangers and are now people I would trust with my life.
I hope to train until I die. And why not? It gives me something more important than mere pleasure; it gives me meaning.