Cats like to hide. That is because they employ something called crypsis to keep themselves safe and sound. A cryptic animal, like a cat, uses its anatomy and behavior to hide from anything which might predate them. It is different from camouflage as this can also be employed by predators and is used by many animals, including big cats, to more effectively attack prey. A house cat uses its natural flexibility to hide in places a predator might not consider. A box is just that. Inside it, a cat feels invisible, and that is exactly how he likes to feel.
Perhaps these exceptions don't quite prove the rule, but let's go with it.
