Teachers who have taught the same subject a long time and reuse the same materials sometimes get bored with the material and find it so easy that they can easily lose perspective on how hard it is when it’s new to you. I will never forget visiting a math teacher’s class when a child asked about # 43 on page 291 (or something). Without looking at her book, the teacher started solving the problem on the board. No explanation, no asking questions of what to do next or why, just writing out the solution. Not only was the solution from memory, the teacher had taught the course for so long she didn’t even need to look at the problem. Wow…
That’s one *very* good reason for using a healthy mix of new problems in every assignment every year (and there are certainly others if the assignments are to be graded).
I have also found that once some teachers choose a textbook, they have done well over 3/4 of the planning and thinking they are going to do about the course. It is distressingly rare that a teacher, particularly one who’s taught a course awhile, asks the questions I think are critical: Why is this topic important? How important is it compared to other topics we might be teaching and why? What level of mastery do we want: exposure, reasonable competence, facility, or mastery? And why? And how much time will it take to get to that level? What are the best assignments to get the students to the point at which I want them to arrive? What’s the cost/benefit ratio of the work I’d be assigning to get there?
Those questions should be asked of every major topic in any course, not just courses that people tend to teach one chapter at a time out of a textbook. Too few people do. And once asked, the questions are rarely asked again except in a minor-modification way.