What makes a kid love a book?

I'm reading a book with a group of 4th graders. Tales of a Fourth Grade Nothing. They love it! But the main thing they love about it is the turtle, named Dribble, who is mentioned on the first page of the book and who is mentioned every now and then throughout the narrative. The turtle is a minor part of book (so far, anyway)--but the kids love the book because of Dribble. Every time the turtle pops up they get all excited and talk about what a great book we're reading. Without Dribble the turtle I'm not sure this particular group of boys would care much about the book.