Why do cats purr?

Cats don't just purr when they are contented. Therefore, you need to find an answer to the question which covers all circumstances under which domestic as purr. Dr Desmond Morris, the celebrated zoologist and animal behaviourist came up with the right answer in 1986. I'm a bit surprised that is not more widely cited so I will mention it here. In fact, I will quote him verbatim if I may:

"A more precise explanation, which fits all cases, is that purring signals a friendly social mood, and it can be given as a signal to, say, a vet from an injured cat indicating the need for friendship, or as a signal to an owner, saying thank you for friendship given". 

I don't think I can explain it any better than that. The point that he is stressing is that domestic cats purr when they are in a veterinarian's clinic, on the consulting table where they are clearly nervous and stressed. And sometimes they purr when they are about to be euthanised by a veterinarian. Clearly under these circumstances domestic cats are not happy and contented. This is why Dr Desmond Morris was obliged to come up with an all-encompassing reason for the production of this sound. He did that. 

There is another answer to the question, "Why do cats purr?" and it is this: there are different reasons for purring depending on the circumstances. It is a flexible sound that means contendness when being petted or when the cat is on a lap and it means a need for companionship when about to be euthanised.

People sometimes ask whether the wild cat species purr. Some do and some don't. The bobcat and lynx can purr and it is a very impressive sound. The big cats can't but they can roar instead. The small wild cat species can purr but when they do so it is a slightly different sound to the gentle and melodious domestic cat purr.