Why do cats knead?

Cats are solitary animals, unlike dogs who live in packs. The only time a cat ever has a close relationship with another cat is its mother. If you adopted your cat when it was young, it probably thinks that you are its mother. The kneading motion is something that the kittens used to do to their mother, presumably to get her attention/affection/ and to get her to feed them.

No one knows for sure, but the most popular explanation is that kittens knead when they suckle in order to increase the milk flow from the mother's tit. It stays for life because the cat gets used to the positive result of this action. Some people say that if a cat kneads it means that they were weaned too early or too late, others disagree, and others think that kneading has nothing to do with suckling at all.

what about cats that have mates on them

Because they have scent glands in their paws.  When they knead, they are putting down their scent, just like when they rub their side against a wall or a person's leg... it's to leave their scent.

Can anyone tell me why cats yawn when you run your fingers down the teeth of a comb