Yahweh realized that everyone in the world was really pretty fucking wicked. All they ever did was think up evil shit all day, every day. And worse, they were probably saying bad things about him, too.
Yahweh knew he wasn't perfect, but he was learning. And one thing he knew for sure is that he was truly sorry he ever made mankind and placed him on the earth. The whole thing gnawed away at his perpetually beating, self-created heart.
So Yahweh said, "I will blot out from the face earth everyone I have created! You know what? Fuck it, I'm gonna kill all the people and all the animals! Who knows what those nasty animals are thinking? Whatever it is, I'm sure it's evil, too. In fact, I'm gonna kill all the people and all the animals and all the creepy-crawly things and all the birds in the heavens. I'm sorry that I ever made any of them!"
Except, of course, for Noah. Yahweh had a weakness for Noah. Noah was no Utnapishtim, but he was generally good. Especially considering the rest of mankind was so fucking evil.
Would it be that big of a deal to spare just Noah?