Cats often purr when they are happy and content. This is especially true when they are being petted or scratched.
Cats also purr when they are feeling content and relaxed. This is often seen in cats who are sleeping or sunbathing.
Cats also purr to communicate with other cats. Purring can be used to greet other cats, to show submission, or to ask for food or attention.
Cats purr at a frequency of about 20-50 hertz. This frequency has been shown to have therapeutic effects on humans and animals.
Purring can also help to bond cats and humans. When a cat purrs, they release pheromones that can make humans feel relaxed and happy.
Kittens purr when they are nursing from their mother. This helps to stimulate the mother's milk production and to create a sense of bonding between mother and kitten.
In rare cases, cats may purr due to a medical condition. For example, cats with hyperthyroidism may purr more often due to the increased metabolic rate.