Actually, there's two questions (although they may have the same answer). 1. Why do you teach?(e.g. this could be because of a sense of obligation; or it's a requirement for grading); and 2. what rewards, returns do you get from teaching? What makes you keep on coming back to teach? You need not be a regular instructor to answer this question.
Without teachers you would have no students Without students you would have no teachers and no art. Edit.. after a bit of searching : http://www.eastcoastaikikai.co.uk/kenshusei.htm this explains a bit about my thoughts on teaching in aikido.
Although I wasn't a regular teacher I used to love teaching and took every opportunity there was to help other students. There's just a certain sense of satisfaction in seeing somebody do well and knowing you genuinely did your best to help them.
