It might be easier to ask the reason why humans do not purr. Purring is among the mysterious things done by cats, and it has a lot to do with feline physiology. Learning more about it makes us realize the better deal that cats got from nature.
Take the wild mother cat as an example. Since her kittens are both blind and deaf, they have to look for their mum (plus the milk bar). It is fortunate that a 2-day old can figure out where the milk is when it is warm and vibrating. When a kitten is 2 days old, this is the time when it starts purring back. This can work for the benefit of the mum, because she will know the whereabouts of her kitten. In case there is danger somewhere, mum’s beady eyes can remain fixed on the hazard, and when she feels the vibrating fur balls against her body, she will know that they are all safe and sound.
This can be compared to chicks and ducklings. Their continuous piping easily catches the attention of passing foxes (and even passing cats). If you want to remain connected with your contacts, but not heard, you usually set your phone to vibrate.
Mother and child bonding are memorable moments to both of them, so probably, cats purr whenever they feel contented – just like when they do the same kneading motion using their forepaws that they used to do to get their mother’s milk. It is not a coincidence that they do this whenever humans stroke them just like a mother’s tongue did when they were young.
However, cats purr not only when they feel happy. They also purr when they feel strong emotions that do not entail adrenaline. They purr whenever they are frustrated, feel pain or when they are about to die.
Following hundreds of years of confusion, scientists have finally put everything together. In essence, a cat filled with emotions receives a reflex coming from the central nervous system, that in turn sends a message to the muscles in the voice box (laryngeal muscles) to tighten, to a certain extent that they vibrate and air enters and goes out of the lungs. For this reason, cats purr constantly, whether they breathe in or out. There are cats, though, that have a missing/damaged laryngeal muscles, so they cannot purr. Nobody really knows if the purring sound is voluntary or involuntary.
Cats live long lives even if they are small in size. This is since (as all cat owners reveal) they do not overly strain themselves. A cat remains well toned and fit because it purrs.
The 25 and 150 Hz frequency helps in developing bones, and this is the reason why cats seldom have dysplasia or osteoporotic conditions commonly found in dogs. Also, it is interesting to know that humans living with cats have a lower blood pressure. This is certainly not caused by the torn curtains or poor toilet habits. Doctors are sure of the benefits associated with cats to permit them to serve as therapy animals in a number of hospitals.
Therefore, why do cats purr? It is simply because they can – just like pumas, raccoons and mountain lions. Humans cannot purr, so the next best thing is to own a cat that purrs. After all, it is not recommended to have a mountain lion around.