You don't know exactly why cats would purr, but they don't only purr when they're having a good time. They also purr when they are injured, or when something hurts, or when they are hungry. In any case, it is believed to be a form of communication for those in the immediate vicinity of the kitten. Demolition is also said to have a healing effect that can be demonstrated. In animal hospitals, it often happens that if there are several kittens in one place and one of them is injured or sick, they will snuggle up and purr. This phenomenon has been called purring therapy. :)