Why do they put their hand in our mouth or nose while breastfeeding ?
Do you know why a baby puts its finger in its mother's mouth ?
-> The baby receives something from its mother and spontaneously wants to give her something in return.
It is called "cross identification" : it identifies itself with its mother in this "give and take".
It is the notion of "mutuality" : he realises that Mummy does something for him, that she gives him something (milk, security, warmth, comfort, etc...) and then the baby has the desire to give something to his mummy too.
He is saying "thank you, I love you 🏼 mummy".