I initially started through fitness, then started to enjoy learning the techniques, language, applications, etc. I continue now purely because it has become a large part of my life. I like the people in my class and in martial arts in general (so far that I have met anyway).
Even though we train in groups and do group kata, etc. It's a very personal and almost selfish thing to do, you have to find your own drive and determination to continue. To repeat time after time after time a technique - to get even remotely like you were shown it requires a certain drive and patience I wasn't aware I had before MA training. I feel less tense these days to, I have a healthy outlet for my frustrations
My 2p.