I'm sure I don't have all the answers to why my sweet ones do what they do, but I've learned that it's important to really try to find out. We see those memes floating around that say something like "they're not giving you a hard time, they're having a hard time."
The first idea that really went against the 80% of parenting advice that's behaviorist in nature came from the Soul of Discipline Course (and from Simplicity Parenting) is the idea of disorientation. Kim John Payne says he has never met a disobedient child, only a disoriented one. He likens children to ships pinging while navigating in the vast sea. We know whom they bounce off, right?
So why do our children feel lost? There is a lot of life pressing in on them, for one thing, and it creates a kind of "soul fever." That's where simplicity can make such a tremendous difference. (I took the course to become a group leader/coach for Simplicity Parenting because it has meant so much to our family.)
Dr. Gordon Neufeld talks a great deal (as regards anxiety) about separation. There are so many ways/levels for a child to feel separated--at least as many as the levels/ways to be attached. That's intense, and their brains aren't prepared to make sense of it (especially at a young age).
Dr. Gabor Maté mentioned in a talk on stress and parenting that we misunderstand the phrase "acting out"--literally, when we act things out, it's because we don't have language for it. If our children experience things they don't understand, they do end up "acting" them "out."
All behavior is a sort of communication. What does it tell us about what's happening in their lives/brains? (And how is our own inner response to that behavior revealing things to us about our own lives/childhoods/brains?)