Because of her early days experiences, a kitten equates being provided for with her mother. As the human caregiver continues to provide for the now adult cat, the cat relates to their human caregiver as their mother.
Her true mom licked her kitten a lot during her earliest days to clean them, which is part of the normal mother-kitten relationship. The kitten regards being licked as coming from their mother. As a consequence, when stroked by a human she regards it as being licked by a giant ‘pseudo-maternal hand’ in the apt words of Dr Desmond Morris.
It is enjoyed and stroking becomes petting, a more varied form of tactile interaction between human and cat. The cat seeks out being petted.