Why they purr. Is a purring cat a contented cat? Not necessarily. Cats in great pain, even dying, often purr load and long. Contented cats also purr, of course but in fact purring signals a friendly social mood and can be given as a sign to, say, a vet from an injured cat indicating the need for frieendship or as a sign to an owner saying thank you for friendship given. Purring first occurs when kittens are only a week old and being sucked by their mother. She purrs to them in turn, assuring them that she is in a cooperative mode. Purring between adult cats and humans is almost certainly derived from his primal parent offspring context.