The Feline Smile
People often consider purrs to be an expression of affection, and it is clearly a communication tool. Cats and kittens rarely purr when they are alone. Purrs are aimed at other cats or people. The purr has been described as the feline equivalent to a smile, which makes great sense. After all, people smile for all kinds of reasons--happiness, nerves, fear--and a smile (or a purr) doesn't necessarily indicate happiness.Purrs rumble at all sorts of occasions, even when Kitty is frightened or in pain. Some behaviorists suggest that the purr is a sign of submission that signals to the other cats and people that, "I offer no threat" or, "comfort me." That may be why the purr is used both in times of contentment to express joy, and during times of stress to relieve tension. When your cat comes to you with kneading paws, and luxurious purrs, perhaps that's her way of calling you "mom."
Some scientists theorize the purr also serves as a healing mechanism. Feline bones heal much more quickly than other mammals, and in human medicine, vibration of similar frequency to the purr appears to speed healing. Could the feline purr be an evolutionary asset for our cats?
As a summary, cats purr because it's a communication tool for them, and has also been described as a smile for cats. Since they can't smile, they purr. Make sense? If not, go here: http://cats.about.com/od/amyshojai/a/cats-purrs.htm
Fun fact: Some cats purr when they are dying. This is mostly because they know they will be out of pain soon, and they are happy to be letting go.