Why do some cats purr loudly and often whilel others do not?

There are only four cats in the world that can roar: The Lion, Jaguar, Tiger and Leopard. These cats have specially-designed larynx's and hyoid bone in the throat that enables them to roar. No other species of cat can roar. Cats roar for various reasons, including territorial proclamation, communication with other members, and anger.

It is now thought that all cats, even "big" cats that can roar, can purr. Big cats, with their specially-designed throats, can only make a purring sound when exhaling, and not in a constant stream like other cats. It is unclear why cats purr, although it is well known that domestic cats purr when content, to show affection or to comfort themselves when sick or in pain.