#17 -- The Redux Principle. Having children guarantees that you will be forced to relive the worst moments of your own childhood.
Case in point: 5th grade. Not my favorite year of school when I was a kid. I had the experience of spending a year engaged in intellectual redundancy, with the distinct feeling that I was ready to move on. This, of course, made the time stretch out into some unholy event horizon of the black hole of public education. I felt like a freak because I had absorbed everything that elementary school had to teach. Yet I was emotionally attached to the comforting surroundings and the regular presence of people who were my own age, even if time has proven the term "friend" has a very fluid meaning.
Now, some decades later, I'm going through 5th grade for a second time with my own children, and suffering through all the conflicting emotions with them. Time has given me the perspective to articulate the problems I had back then (see the above paragraph) so I figured I had some parental insight. However, that ignores the guiding principle all children live by (which is available only to kids as their own super-secret list that you have to figure out through guesswork): parents are clearly imbeciles and any insight they have is the insight of a feeble-minded hamster which will routinely eat its own poop.
The end result: you are finally equipped to deal with 5th grade and the boredom, bullying, cliques, teachers, administrators, ceaseless activities, fear, exhilaration, and all the issues that seem so momentous to pre-teens, but the kids don't care in the least that you can help them. Which leaves you feeling just as powerless as doing it the first time around, except you have to watch the people you love the most suffer through everything, when your fiercest wish is to spare them that suffering. You have to be patient (not my long suit) and watch them figure it out themselves.
Parenting is not for the faint of heart.