Ok, I'll analyze the content...

Max loses control and allows his wild side to take control (kids do this a lot), he enjoys it (kids also do this, as do some adults). He takes control of his wild side, which makes him feel powerful, and he enjoys that as well. When he is finished with all of that, he is redeemed and still loved, which he finds reassuring after having the experience of everything getting out of control.

It's very psychological, and it speaks directly to things that are under the surface. It is very effective for this reason, and children find it satisfying and reassuring.

I first read this book while taking a child development class, and it was very highly recommended by my teacher.