"The reason birds can fly and we can't is simply because they have perfect faith, for to have faith is to have wings."
The narrator convinces David that the only difference between very young babies and birds is this "perfect fate." Babies are actually just birds who have doubted themselves and now need the support of older humans. David takes this point so seriously that he does tap into a belief in himself that allows him to fly.
"When you were a bird you knew the fairies pretty well, and you remember a good deal about them in your babyhood, which it is a great pity you can't write down, for gradually you forget, and I have heard of children who declared that they had never once seen a fairy."
Noticing David's fascination with fairy stories, the narrator tells him more and more about when David used to know the fairies, before he was a baby. He's intentionally linking the fantastic with youth, desiring to persuade the boy to reject growing up. In this way fairyland becomes a link to innocence and perfection, something which David has lost already, though he is still a young child.
"I have seen him climbing a tree while she stood beneath him in unutterable anguish; she had to let him climb, for boys must be brave, but I am sure that, as she watched him, she fell from every branch."
Mary loves her son more than anything. She would gladly spare him any pain, but she is a responsible mother who understands that David must discover things on his own. This reservation spares Mary none of the pain of watching her son in danger, but she does her best to balance preservation and discovery. More importantly, the narrator observes and understands that Mary truly loves her son.
"She has been putting qualities into David, altering him, turning him forever on a lathe since the day she first knew him, and indeed long before, and all so deftly that he is still called a child of nature."
The narrator observes jealously how Mary has such influence over her son. She has imparted her beliefs into him, added some ideas to his blank canvas. While she has good intentions, she has deliberately shaped the boy. This observation is nearly unbearable for the narrator, who desires to possess David in his pure innocence, in order to prevent any alteration from adult society which would turn the boy against him.
