Male cats knead with all four paws primarily because they kneaded as kittens to get more milk; this action is a way of remembering their mother. Cats will also knead if they have a close bond with their owner, indicating that the cat considers the owner its mother. Don't interrupt them while they knead, and occasional petting will make them love their owner even more.
In addition, cats have sweat glands on their paw pads that secrete a liquid with a distinctive scent. Indoors, especially during mating season, cats may knead their paws, leaving this scented liquid in various corners of the house. This serves both as a way of declaring their territory to other cats and as a way for their owners to smell their scent and feel safe.
