This is a common question that we receive during an appointment, as well as in general conversation. There are two ways to answer this question and we will attempt to do so below.
The first approach is to answer with a clinical, or physiologic, response. That simply means to answer the "HOW" portion of the question. What is going on physically with my cat that makes him/ her purr? The easiest way to answer this is that purring is generated by vibrations of the vocal cords. These vibrations cause the audible sound that you hear and the vibration that you are able to feel. The vibration occurs as the result of brain stimulation, which sends a neurologic response to vocal cords.
The second answer to the question is why your cat actually purrs. Another way to state this is "What does it mean when my cat purrs?" One common response is that a cat purrs when she/ he is happy. Cats often purr when they are in contact with their owner. They tend to purr when they are comfortable or content. You may notice that your cat purrs in response to being pet or stroked. Cats also purr when injured or sick. Some people believe that the purring my be associated with greater healing. Cats sometime purr when they are scared. Cat purr when nursing their kittens. Cats also purr when nervous. One additional cause of purring is hunger. Some cats will purr in order to try to get food from their owners.