A new course is like a new baby. You have to feed it, bathe it, calm it, put it to bed, keep it appropriately entertained and distracted. You worry about it a lot, especially when it gets sick, to the point of complete distraction from everything else in your life (your job, showering, etc).
The second time you teach a class, it is like having a toddler. It is slightly more capable, but you need to worry about it choking on errant objects, not looking both ways before crossing the street, and ensuring fried potatoes are not the only vegetable it eats.
The third time you teach a class, it is like having a kindergardener. You worry about it occasionally, like when fights or cdiff break out at school, but overall you are considerably more relaxed.
It is around year three or four that you start to get a little heartsad. You miss the excitement you felt when you found That Perfect Example, or The Hilarious Video, or even that time you discovered those amazing lecture notes from the University of Alburquerque on set theory with the two pigs. The course materials don't need you as much as they used to.
So you putter around, tweaking things here and there, while idly toying with the idea that next semester, by golly, you're going to prep a new class. Just as nature makes parents forget the trauma of pregnancy and the agony of not sleeping for 2-3 years, academia, too makes us forget the birth of a course.