I think what happens is if everything is going well we know that it can’t last forever so we prepare ourselves mentally for a letdown. How much time you spend thinking about it depends on your personality. I almost never think about big things going wrong. However when I get to work hitting all green lights, I find a parking space reght next to my classroom, find the copy machine free, don’t have any surly kids and finish up by 3:30, I just know something is hanging over my head. Having minor challenges actually offsets that little frisson of threat you get on a perfect day.