The most common reason that cats purr is to show happiness or contentment. However, research has shown that purring also appears to have a calming or healing effect, so cats may also purr when they are frightened or ill.
According to WebMD, Kelly Morgan, a clinical instructor at the Chicago Center for Veterinary Medicine, stated that a cat purring is equivalent to a human smiling. Humans tend to smile when they are happy, nervous and even when they want something. Dr. Karen McComb also conducted a study that shows cats sometimes add vocalization in order to receive a reaction from humans.