The fundamental task of a teacher, beyond all others (at the practical level), is to take something complicated and make it simple enough for students to apprehend at their level of capacity.
But Darla Sowders is right, it doesn't come naturally. It requires genuine knowledge of the discipline being taught, love of the student, and an understanding of how people learn. Moreover, it requires a specific understanding of the student's readiness for a given lesson.
None of these things are developed by a subject or textbook approach to learning.