I never wanted to teach. I thought teachers were mostly people that couldn't make it in the "real world", and had gotten forced into academia to keep from starving.
In my senior year of high school, a friend of mine bugged me to teach her son until I finally said yes. I told her I would be a crappy teacher, and that she should go try and find somebody who did this kind of thing for a living. When she finally convinced me, I figured I'd give lessons for a few weeks, the son would lose interest, and I'd be able to go back to my old routine.
Five years later, I'm still teaching. I've learned that teaching is one of the hardest jobs on the face the planet, and also one of the most rewarding. It requires self-sacrifice, and the ability to communicate the essence of the art form you're teaching in a practical, sensible, entertaining way.
It also requires you to keep learning. If you stop, your students pass your knowledge level surprisingly quickly. It's one of the biggest advantages of teaching- you're forced to learn new things constantly.
Every single musician should be a teacher. Not just because you need extra pay, but because you need to share what you've been given with others. We often forget just what a gift we've been given with music, and don't realize that we have the ability to give it to dozens of people around us every day. Don't waste that amazing opportunity.
The next time someone asks you to teach them, say yes. You might be surprised what you learn.