A cat’s purr is often interpreted as a sign of pleasure, but that’s not always the case. Cats emit this peculiar vibration through very high frequency movements (25 times per second) using the muscles of the larynx, opening and closing the glottis and letting out air in quick pulses, producing a very distinctive sound. Cats of all ages, breeds, size, weight, and sex have the ability to purr, and they share this characteristic with other members of the cat family such as pumas, cheetahs, and ocelots.
Cats use purring as a means of communication among themselves or with humans, especially to keep in touch (there are even ethologists who have described different types of purring: to declare their well-being, to request something, to calm themselves down...). For example, a pleasant situation such as sitting on their owners’ laps, receiving body heat and affection, gives cats pleasure and therefore causes them to emit these sounds to communicate and prolong the situation. They also purr in the presence of other cats to maintain good relationships, for example when they feed their litters or are being fed themselves.
Surprisingly, cats may also purr in situations of pain or stress, so it’s inaccurate to make the generalisation that cats only purr out of pleasure.