It isn't just that they play with anything. A box is special. When I first gave my puppy a carboard box (6 years ago) she didn't know what to do with it. Then I tore it a little and something clicked. The box, unlike everything else in the human world, was expendable. There is nothing that she could do to the box that would upset anyone. Rip it. Sleep in it. Carry it around. Or forget about it and do something else. Another one will appear eventually. That's freedom. Every other thing either belongs to mom/dad or came from them and so must be used properly. Kids enjoy boxes for exactly the same reasons and in much the same ways as puppies.