It's one of the most dreaded questions in the world. Well, at least if you're a parent, at any rate. I mean, what business is it of your kid's to know "why?" she needs to do something, right? Or "why?" she needs to do it the way you're telling her to. Or "why?" she needs to go to bed RIGHT NOW! And, sometimes, all you're left with is "because I said so!"
Of course, teachers have the same problem. And it can be worse for them, because, often, they don't actually know the answer to "why?" All they know is that the book says so or the school board says so or whatever, but those amount to the same "I said so" to a kid, and, for a kid asking "why?", "I said so" is rarely enough.
I learned at a fairly young age to not ask "why?" No one ever had answers for me, so I learned to figure them out on my own. I didn't stop asking the question, though, which is what I think a lot of people do. When faced with inadequate answers, they just don't question, which is how I think most people like it.
And, actually, probably how most authors like it.
As an author, "why?" is the worst question ever!
"Why didn't the they just tell them what was going on?"
"Why is there only one city on the whole planet?" (Come on, Star Trek, really?)
"Why didn't he just kill him when he had the chance?" (Because he was killing everyone else, right?)
The problem with the answer is that it's usually an "out of story" answer. Like, "he didn't just kill him because, then, the story would have been over." Or "there was only one city on the whole planet because it would have been too complicated if there had been a whole planet of people to deal with." But out-of-story answers don't work for those questions within the story, which is why it's the author's job to anticipate the "why?"s and provide answers for them within the story. It's why you have so many super heroes with personal codes against killing. That provides the answer to that particular "why?"
The problem is that the "why?"s frequently have to do with a character's (or characters') motivation(s), and that, when dealing with science fiction, can often be at odds with the IDEA. So let's take Looper (you can read my review of it here) as an example:
In Looper, the premise is that the mob of the future has a time machine and they use it to dispose of pre-dead bodies.
Really? That's it. That's the best use they could come up with for a time machine? Which is the problem, because the IDEA is that a guy in the "present" is sent his future self to kill. Wouldn't that make a great story? And, as long as you don't ask "why?", it does make a great story. However, as soon as you say, "Why doesn't the mob in the future just use the time machine to take over the world?" or "Why doesn't the mob of the future use the time machine to invest in all of the right companies?" or "Why doesn't the mob of the future use the time machine to win every lottery ever?" As soon as you ask one of those questions, the story falls apart. And, sorry, the answer "they didn't think of it" just doesn't cut it.
And I'm not even going to start on the scene where the future guy loses his body parts in the present as his present self is tortured and then killed, because there's no way you can answer the "why was that guy there to begin with if he was already dead?" question. Sure, it's cool and freaky to watch as he loses his fingers and stuff, but "cool and freaky" doesn't answer "why?"
Then there's the great "why?" of the Star Trek reboot: "Why did Nero just hang around in space for a quarter of a century doing nothing?" And the obvious answer is that Abrams wanted Nero to kill Kirk's dad but had nothing else for him to do until Spock showed up, so he just had him sit and brood with his thumb up his ass for 25 or so years.
And I could go on but, really, why?
As authors, it's our job to anticipate the places where our readers (or viewers) will say "why?" and weave those answers into the story. Sure, a lot of people, maybe even most people, are not going to ever say "why?", but that "why?" will kill a good or fun idea for those of us that do ask. And, no, telling us not to ask or to "just go with it" doesn't work either. That's a cop out. And irresponsible. And, well, sloppy story telling. Be asking yourself that "why?" question all the way through while you're writing and, anytime you don't have an answer to the question, figure it out. Or take that part out. Something.
And just by the way, long ago I did tell my kids that they could always ask me "why?" I'm telling them something. Man, that's a hard thing to live up to. My daughter, most of all, really holds me to it and, when I try to wiggle out of it, she will say to me, "You said I could always ask you why." The best I can do is delay, too, by saying something like, "Right now, you just need to do what I'm telling you to do. Later, ask me again, and I'll explain it to you." And she does, too.