Cats are not naturally social in the way dogs are, and multi-cat households frequently generate chronic social stress that causes welfare problems for cats that owners often fail to recognise.
Multi-cat households create welfare challenges because cats have not been domesticated to tolerate living in close proximity to unrelated conspecifics in the way that dogs have been. Forced cohabitation with cats they have not chosen to associate with causes chronic stress for many cats, expressed through subtle behavioural changes that owners misinterpret or miss entirely. A cat that avoids rooms where another cat is present, or that over-grooms, or that urinates outside the tray, is communicating chronic social stress. The welfare solution requires providing sufficient space, resources, and exit routes that cats can avoid each other when they choose, recreating the social distance that outdoor cats maintain naturally.