It is simple, people like dogs or cats. Cujo was a good dog. Then he turned bad. Two extremes. People can't handle that. It's like a good person turning rabid. I love--in the book and the movie-- how the boy goes something like, "Cujo, where are you boy, is that you Cujo." and Cujo wanted to rip his throat out, but, because the boy was nice and pure, evil couldn't handle it.
