Why do cats purr when they sleep?
Cats purr are mostly associated with the expression of their feelings. It could be because of joy, fear, calmness etc.
Most of the time cats purr while sleeping, it shows their contentment and relaxation. It occasionally occurs when they are sleeping on their owner's lap or when cuddled.
Another reason for it can be good dreams or terrible dreams. Cat also purr to express feelings of their dreams.