Why does a bird sing?

Pardon the pun but this lovely story quoted in Supercoach by Michael Neill struck a chord with me:

A teacher who had received much acclaim for his insights and discourses into the nature of the universe was asked by one of his students what difference he hoped to make in the world through his teaching.

After a few moments' thought, the teacher replled that he had no such hopes.

"Those who can truly hear what I have to say don't really need me to say it; those who can't hear could listen until I was hoarse and could no longer speak without changing in the slightest."

The student was confused.

"But if you can't make a difference with your ideas why do you teach at all?"

"Why does a bird sing?"