Admit it: the book is funny. That, and the deus ex machine) ending just brought it all together so there was a happy ending with Bob, Estelle, Lucy, and Luke as the perfect couples. That's classic comedy right there.
But it's not just light-hearted fun and games. This is satire—comedy with a bite. Like those jokes about God that happen every other paragraph:
"Your clothes, O Holy Master of All." Mr. B bows and hands him a sweatshirt with a large sporting-goods logo on it, which Bob dutifully pulls over his head. He hasn't changed out of the same T-shirt in what might be a week now. (5.12)
Or this one that riffs on a Bible passage:
And then Bob went on to create every creeping thing, and some that leapt and climbed and slithered and tunneled as well, and he told them to be frantic and multiply, which they did by the most gobsmackingly weird mechanism Mr. B had ever observed, one that slightly embarrassed him as well. (6.14)
Compare that to the original text: "And God blessed them. And God said to them, 'Be fruitful and multiply and fill the earth and subdue it and have dominion over the fish of the sea and over the birds of the heavens and over every living thing that moves on the earth.'" (English Standard Version, Genesis 1:28)
What a way to turn the Bible into a sex joke, huh? Making fun of an original work like that makes this totally a parody. Easy peasy.
Young adult novels: aimed at teenagers, staring teenagers, and mostly written by old people. How are they directed at teenagers? Well, think Harry Potter and Twilight: magic, romance, a little sexiness, and intrigue—not to mention plots that deal with your typical growing up stuff, like parents, boyfriends, and mood swings.
Hey look at that, There Is No Dog has all of those! Well, except maybe for the magic.
By the way, it's useful to remember that young adult novels are usually, although not always, a bit less hardcore than their grownup counterparts. That might explain why There Is No Dog tries to keep an upbeat and funny attitude while talking about some pretty deep issues. If this weren't a young adult novel, things might have gotten much, much darker.