I'm not a professional teacher, but I enjoy sharing what I know to who I know. It's one of those things that gives me a sense of self-worth.
I think my first motivation would be to help a person grow, secondly to help the person to understand. In highschool, and sometimes in college, I was called upon to be a teachers aid of sorts. One thing I cherished was to be able to break down what I knew into a palatable form so whoever I was talking with could understand it on the same level as everyone else and rise from there. I always held firm to the belief that everyone had the same potential, and it was my responsibility to magnify their abilities. I wasn't perfect; often times I found that if I were to place myself in the role of 'teacher', it came off as condescending. Oddly, if I took a road of humility, it sometimes came off as even more condescending.
The role of "teacher" is automatically a position of authority, so with that, you need experience - you need to become 'one' with the curriculum so your ego doesn't twist the process.
Recently it's been harder to "break down" material because my own vocabulary is so particular that I would rather be precise than lose the meaning of what I'm talking about.
Also, even though I've never been a teacher, I figure I would be kind of shitty at the disciplinary aspects.