It is normal for a cat to purr all the time. This is a sign that the cat is in a good mood and feels comfortable. This is caused by the vibration of the cat's false vocal cords. Cats are more likely to purr when you pet them, massage them, or feed them snacks, and adult cats like to purr more than kittens. If a kitten purrs all the time and the sound is abnormal, it may be caused by respiratory disease. Need to be sent to hospital for examination.