Whether it's the bedspread, carpet or a companion, kneading is a very common behaviour for cats. But why do they do it? 

Answer:
Well there are all kinds of theories, some say kneading originates from treading down grass and foliage to make a temporary nest, or from stimulating a mother's teat to secrete milk. In fact, whilst kneading is a very common behaviour for cats, no one has determined why they do it.  Like people have their habits, it appears kneading is just that.  One thing researchers do know, is that when a cat is kneading it is relaxed and comfortable.  If you are on the other end of the kneading, just think your cat is relaxed and happy to be with you!