They recall the maternal tongue.
The touches from a petting hand remind animals of the caresses of the maternal tongue used during licking. These sensations create an emotional bond between the offspring and the mother, facilitating the release of 'happiness hormones.'
This is most vividly expressed in cats: when they are petted, they often start to 'knead' with their front paws, mimicking the movements that a kitten uses to stimulate the mother's milk glands during feeding. Human affection elicits such a response even in adult pets, as during the domestication process, humans unconsciously selected individuals with certain infantile traits — in particular, those who respond to tenderness.
