For the good of the other, that is the answer to our question. Or is it? What does that even mean? In his Nicomachean Ethics, Aristotle explains what friendship is. Perhaps understanding friendship might help us to understand the relationship between teacher and student, even if the relationship between a teacher and his student is not one we might typically describe as a friendship.
For Aristotle, that student of Plato and therefore Socrates, there are three kinds of friendship: a friendship of utility, a friendship of pleasure, and pure friendship.