Cats purr for various reasons, and one of them is to express contentment and happiness when they are being petted. Purring is a way for cats to communicate with humans and other cats, and it signifies a sense of calm and relaxation. Kittens start purring when they are only a few days old as a way to communicate and bond with their mothers. Sometimes, cats purr when they are hungry or want attention. Purring can also be a way for cats to soothe themselves when they are stressed or injured. When a cat purrs while being petted, it is a sign that they feel safe and happy with their human companion. Therefore, when a cat purrs while being petted, it means that they are content and enjoying the attention.