It's a kitten bonding response. Bottle babies often suckle whoever feeds them. If you are lucky they outgrow it. I've got 2 that never outgrew it. Methos is 14 and Puddy is 3 and they both will grab a bit of shirt (or skin if I'm not quick enough to prevent it) and do the sucky kneady routine till they fall asleep. As long as you don't mind the kitty spit it is a behaviour that comforts them.