1. Because I am a really awesome writer.
Well, that may be true. And it's important that you think you are good if you want to get anywhere, because if you think you suck, then your lack of confidence will shine through to prospective agents and editors. But you need proof that you are awesome. You need people (people who really know something, like writing teachers and other writers -- not just your mom or best friend) who tell you that your stories are good (after a LOT of work). Just looking at your own words and thinking you're good enough for a book deal is not enough to make it so.
2. Because children's books stink and I can write a better one.
Yes, there are children's books that stink. But there are a lot, A LOT, of amazing ones. If you think you are the next Mo Willems, by all means, get cracking. But don't knock the field you want to enter by debasing the work the precedes you.
3. Because it's easy.
This one makes writers laugh. Writing is many things, but it ain't easy. Picture books may be less than 1,000 words, but writing a great one is intensely difficult. Every word counts. Just because a novel has a young audience doesn't make it any easier to write. In fact, many writers find it difficult to pare their work down to a reasonable word count.
4. Because I want to be rich.
MWWWWAAAAHAHAHA. That's a good one. I compare writers with journalists. Something insane like the top 2 percent make all the money. The rest of us have day jobs.
5. Because I want to educate children.
This isn't necessarily a bad thing, but unless you are writing textbooks, it can't be the sole reason you want to be a writer. If you are writing children's books to purely educate, then you are probably coming across as preachy. And in today's market, that is annoying.
So if these are all wrong answers, then what is the right one?
Because I must.
Sure, that sounds a little cheesy. And you can take what I say with a grain of salt … After all, where's my published book? But seriously, you write because you have to. Because these stories are IN you. Because when you go to sleep at night, your characters haunt you and beg to be laid raw on the page. You write because you can't NOT write.
No, this reason does not guarantee publication. It doesn't guarantee anything, other than the fact that you take your creativity seriously. In my humble opinion, that's the best place to start.