Why do cats love sitting in boxes?

Because an enclosed space satisfies several things a cat wants at once: security, warmth and a good ambush position.

Security is the main driver. Cats are both predator and prey, and a box removes most of the directions a threat could come from. In an enclosed space with solid sides, a cat only needs to watch one opening. Research in animal shelters found that cats given boxes adapted to a new environment measurably faster and showed lower stress than those without — a finding with practical welfare implications, not just a curiosity.

Warmth matters more than people expect. A cat's thermoneutral zone — the temperature range where it is comfortable without expending energy on regulation — is around 30–36°C, noticeably warmer than most people keep their homes. A cardboard box is an insulator and traps body heat, so it is genuinely warmer inside.

Ambush. Cats hunt from concealment, and a box provides exactly the kind of hide a cat's instincts are built around. The same instinct drives the behaviour of watching from inside and pouncing at anything passing.

Stress reduction appears to be real and measurable, which is why boxes and covered beds are recommended for cats in unfamiliar or stressful situations, including vet visits and house moves.

The curious extension is that cats will sit in a square taped on the floor with no physical sides at all. Studies have found cats settle inside two-dimensional outlines, and even inside illusory contours that only suggest a square. That suggests the preference is partly visual and cognitive rather than purely about physical enclosure.

If a cat is hiding excessively rather than simply resting, that is a different matter and worth a vet's attention.