There is no doubt in my mind that the greatest joy of teaching comes from seeing something you can't control happen: a soul transformed or nourished in a way that leads to visible growth in thoughts and deeds. I can't imagine what would tempt a person to teach if he didn't expect to see this happen.
That is why the CiRCE apprenticeship is one of my favorite activities. After three years of immersion in classical mentoring, all or at least most of the teachers will testify that it did them good. Some will tell you they were transformed.