I regularly see the following quote "those who can, do; those who can't, teach." The quote I find to be an interesting juxtaposition when read together with this quote "a teacher affects eternity; no one can tell where his [or her] influence stops."
The first is attributed to George Bernard Shaw and the second to Henry Adams. Perhaps this is interesting because Shaw was one of the founders of the London School of Economics and Political Science.
The quotes resonate with me as I am about to embark on a Masters degree that will take me away from what I have been doing and place me fair and square within the ranks of educators. I have always believed that teachers are the most influential people in our lives after our parents. So, I agree with the sentiments expressed by Adams (although I added the [or her] to the original quote).
However, I am also tempted to tinker with Shaw's quote so that it reads, those that can teach, do; and those that can't, do something else. My version probably will never be all that well-received or widely used, but I believe that you really have to want to teach in order to be a teacher, because if you are not committed to the cause then you will never do it well.
Such is life.