Why do cats purr?
The Real Answer
Cats purr by rapidly contracting muscles in their larynx (voice box) 25 to 150 times per second, which vibrates their vocal cords. They purr when content, but also when stressed, injured, or giving birth — so it's not purely a happiness indicator. Scientists believe purring may be self-soothing, a way to communicate with kittens, or even to promote bone healing, since the vibration frequency overlaps with frequencies known to stimulate bone growth.
Your cat has installed a motor inside its throat that it can turn on at will, and you've spent years trying to decode whether it means 'I love you' or 'I'm three seconds away from knocking your coffee off the table.' It means both. Your cat is just vibrating through its moral ambiguity.
Why People Ask This
People ask this because purring is the one thing cats do that seems explicitly designed to manipulate humans emotionally, and we fall for it every single time. What makes it weirder is that cats purr when they're in pain, which means you could be petting a cat while it's actively suffering and you'd have no idea — the very sound we've trained ourselves to interpret as contentment could mean your cat is one litter box away from a vet visit.