As a father you will inevitably experience the "why" stage. This is the point in your child's life where the answer to everything is "why"? A "Why" session will go on for an hour if you let it, and I suspect most parents find them frustrating because at some point it bottoms out. At some point you have to admit to your child, and to yourself, that you don't know why. Many patient fathers find themselves impersonating Bruce Hornsby and singing That's just the way it is. Others with the courage to admit defeat will answer "I don't know", but in order to save face might offer "nobody knows". Whatever your answer is, these tests of endurance serve a purpose. They remind you that you don't have all the answers, and thats humbling.
Asking yourself why is a great way to unwant things. When you find yourself lusting after that Tesla or ripped abs or something, ask why. Why do I want those abs? Because that'll make me look more attractive. Why do you think that? Because that's what attractive men seem to have. Why? Because thats what all the handsome fellas in action movies seem to have. Why? You can do this perhaps a dozen times, and at some point the root of your why's is always something inane. You could keep going but you'll quickly become embarrassed with yourself and move on to something else.
This tactic is dangerous though. You can talk yourself out of unwanting many things that most would consider productive. Why do I want to read that book? Why do I want to write that novel? Why do I want anything for myself at all? Be careful where you tread in your unwanting, or alternately have a very solid base to fall back on. I don't mean a purpose, just a backstop where most of your most positive why's land. Without a backstop you'll find yourself nihilistically doing nothing much at all, because you failed to come up with a reason why and instead find yourself sitting still in a darkened room. Why did I write this short essay?
In corporations and conflict resolution it's common to exercise something called the 5 whys. Use in the context of getting to the root of a problem, the point is to ask why until you get to some soluble level of depth without descending into the inevitable "because the big bang occurred" or "because he said let there be light". Even that isn't the root though. One can easily ask why again, and the half-truth your offered your daughter at the beginning of this seems valid. Nobody knows. Mu.
In any case 5 appears to be a level of depth useful enough to solve most corporate problems. It may not be deep enough to get to the root of your personal desires and wants, but try it until you get a satisfactory answer from yourself.
In the end, why always bottoms out. It's like the Arab proverb about baking a cake. In order to truly bake a cake from scratch you must first create the universe. In order to truly understand why you must first create the universe. Maybe you don't need to know why. Maybe you just need a cake.