Because Ma can’t even conceive that a grown man could be sensitive and loving, she makes an absurd request: that the narrator remain six for the rest of his life, thereby preserving his innocence and kindness. This, however, is impossible, since there’s no way to keep a person from growing up and leaving childhood behind. Indeed, that the narrator’s attempt to express his love ultimately backfires and causes Ma to lash out at him signifies just how impossible her request really is.