It's the kids. They're why I haven't lost my mind teaching the same lesson five times every day for sixteen years. I take full advantage of our skill-based language arts curriculum and allow students to build and exercise their skills in ways of their own choosing. Their reading is independently chosen and they have a lot of latitude in terms of topic and genre for their required writing. That way, not only is every year different, but within any given year, each class is different, too.
There's something else. Even when I do recycle an exercise or an assignment, unless I change it a little with my current class in mind, it never works as well. Often I'm disappointed when I re-use an activity that students loved the year before only to have it fall a little flat with a different bunch of kids.
There's that element of zeitgeist that definitely influences group dynamics, and if you can ride that wave, you probably won't burn out. In fact, you might love your job for many years. Hey hey, my my.