My instructor starting teaching just to try and spread the art. At the time, no one was teaching the style on the east coast of the U.S. Since he was in the U.S. Air Force at the time, he didn't need to do it for the money.
I agree with Gou that if you're doing it for the money, you're doing it for all the wrong reasons. While it is possible to actually make a living teaching the martial arts, it doesn't happen often. I've seen more martial arts schools fail than succeed. Some, because the teacher just didn't know anything. Some, because the teacher knew his art, but didn't know how to teach it. Many fail because martial artists usually aren't businessmen first.
I'd like to do it just to spread the art and to keep my skills improving. I've always said that if you can't teach it, then you don't know it. Therefore, teaching the art will ensure that I know the art, as well as give me better understanding of it as I teach it over the years and discover new aspects of the system.
Babbling again. I need a gag.
Cthulhu