This is, I believe, the crux of the message of the child-training part of the NGJ ministries. This is, at least, what stands out to me the most and has influenced my parenting in the deepest way. It is also something that I think a lot of other parenting books don't address in quite the same way. I appreciate also that the next paragraph goes on to address the practical application of this principle.
You say, "But my kids destroy my joy!" I am sure it's mutual. Without aggressive, deliberate child training techniques you kids will be unruly, and your home will be disorderly-- sometimes explosive. You will be unhappy, short, rude, and a gripe. If someone were to ask your kids if you are joyful, what would they answer?
So notice that the key part of implementing Joyful Parenting is to have "aggressive, deliberate child training techniques". Unruly and untrained children destroy the peace and joy of a home, but the blame still lies on the shoulders of the joyless, un-prepared parents. Joyful parents with a plan to train their children in peace and joy create an environment where joy can flourish and grow.
I mean, you can talk all you want about keeping joy in your home, but if your kids are rude, sassy and disobedient, regularly throwing fits and with a complete lack of self-control, then you're fighting an up-hill battle.
On the flip side, you can have little automatons, that obey without question every whim of their autocratic parents-- but without joy, you're going to see their back-ends in the far distance as soon as they turn 18. And none of us wants that.
So the bottom line is that a joyful attitude in the parents must walk hand-in-hand with a positive, pro-active (as opposed to re-active) approach to training your children in righteousness.