When a cat kneads its paws, it's expressing its love for its owner. When a cat sees its owner as its mother, it will knead its paws. Cats only knead when they feel satisfied with their owner, so there's no need to stop it. 
If a cat is in heat, it will also exhibit excessive kneading behavior, as it desires a mate. In particular, female cats will become very clingy and will often knead soft surfaces such as sofas and beds.
Cats also use kneading behavior to assert their dominance, especially when there is a new companion in the household. They will use their paws to secrete a special scent and knead to let the new companion know that this is their location.
