Why do cats pur when they are happy?

Cats purr as a way to communicate, this usually means they're happy but sometimes a purring cat can mean a problem. You can usually tell if its a good purr or bad by when it starts purring, if you are grooming it then it is happy but if it does it for no reason (you haven't touched, talked or been seen by the cat) then thre may be an underlying problem. How the cat purrs still remains a mystery, the cat doesn't have any more (or less) in their body than any other non-purring species. It is hypothesised though. They produce the purring noise by fast twitching the muscles in their larynx, that rapidly expand and contract the glottis. This then causes air vibrations during inhalation and exhalation.