Why do cats knead (slowly claw a solid surface, almost like they're giving a massage)?

Kneading can have several meanings depending on the age of the cat. If it is a young kitten, that is how they stimulate their mother to get milk, so, the kitten could either have been taken away from its mother too soon & was not broken of this habit. Another way is that the cat likes the surface & is marking it as its own. Cats have scent glands on their paws that they use to mark their territory. There are a few other reasons, but those are the main two.