Boxes satisfy cats' instincts to seek cover and watch the world privately, feeling protected while they observe.

 Boxes contain cats' body heat, creating a comfy nest. Cats conserve energy by lounging somewhere warm and enclosed.

Cats enjoy crawling in and out of boxes during play. They bat boxes around and pop out to "surprise attack" toys or people.  

 Boxes trigger mock hunting behaviors. Cats pretend harmless items are prey when batting playfully at objects in boxes.

 Rubbing on boxes leaves scent cues that claim objects and areas as their own private property.  

 Entering an enclosed space eases anxiety, letting shy or scared cats retreat and relax during overwhelming times.