This happens to me and friends of mine who write for children quite a lot. Someone praises one of our books, then -- in an encouraging, coaxing tone -- says something like,
"Why don't you write an adult book? I'm sure you could do it."
This is meant as a compliment, I know, but I find the idea that children's books are training wheels preparing the author for the real two-wheeler insulting! I never have a gracious answer, either. To respond as though I think it's a compliment is impossible. To say what I think -- that children's books are actually HARDER to write than adult books and more important, too -- and then launch into a speech about why, would be rude and boring.
So I usually just say (truthfully) that maybe someday I will. I've wanted to be an author since I was four, but by the time I was a teenager, always imagined my books would be adult books (and great classics, of course). I don't know why I started writing for children; it just happened. I never planned it. It's true that I think I was better at being a child than I am at being an adult (who isn't?) -- but leading your adult life expertly -- being good at handling money, jobs, romantic relationships, blah blah blah isn't a requirement for writing for adults (think of Scott Fitzgerald!); and a happy childhood isn't a requirement for writing for kids, either. Just the opposite, probably.
So why DO I write for children, not adults? In some ways, I find the form limiting -- you can do things in an adult novel that you can't do in a middle-grade one (change the point of view, wander around in time, deal with social class and other things that children don't find very interesting)....but still, I write for middle grade readers.
Maybe it comes down to feeling like I have more to say to children than to adults, and that children are interested and will GET IT in a way most adults won't. I was once at a dinner party and someone asked what I was writing -- instead of giving the sound bite, which is usually what adults want when they ask a question, I gave a detailed description of the first chapter.
I could tell that the adults were bored, I knew I should stop talking, but I COULDN'T. Or didn't. The children at the table, though, were listening, their eyes fixed on my face -- expressionless, but listening. When I was done, the adults hurriedly returned to more interesting topics; the kids were silent, until, after what seemed like a long time but was probably really only a few seconds, the oldest --a girl -- looked at me and said, intensely, without smiling:
"That is SO funny."
That's the closest I can come to an answer. Why do you write YA or middle grade?