As a college professor I have two primary responsibilities to my students; the first is to provide them with instruction in the subject matter content, and the second is to measure whether or not they have mastered it.  Their subject matter mastery is usually measured in the form of a test—and it is always my desire that they will earn a high grade on it.  I know, however, for it to be a valid measurement that the test must be difficult enough to align with their academic maturity—I do them no favors if it is too easy, and it is not a reliable measurement if it is too difficult.