What doctors say
According to New York veterinarian Dr. Rachel Barrack, this kneading isn’t because your cat is secretly plotting to turn you into a loaf of bread. It’s actually pretty sweet. The doctor says that kittens start kneading on their mothers’ bodies when they nurse. When they grow older and more independent, this practice stays with them and they start kneading on their siblings, owners, or soft bedding.
As kittens, this kneading practice is not just meant to show affection, it has a practical use as well — little kittens massage their mother’s teats in order to encourage lactation and help the milk they drink come out more easily. Kittens will often purr while nursing — they’re feeding on the best food possible, close to a calming source of comfort. This is also why your cat might purr when kneading on your stomach! Dr. Barrack also adds that kneading has a special effect on other cats. Kneading soothes and entrances them. This is the kind of calming effect your cat is so graciously trying to have on you.
There could be more reasons
Another theory about this kneading practice is a little less more practical than heartwarming. In the wild, cats knead their sleeping area (dirt or leaves) to make it softer and more comfortable. This habit simply carried through to their domesticated lives.
An additional explanation as to why your cat sees you as a piece of playdough is biological but still adorable. Cat paws have scent glands on them, and when your cat kneads you, it marks you with its scent, letting everyone know you two are connected. How sweet!