How we cite our quotes:
God knows how infantine the memory may have been, that was awakened within me by the sound of my mother's voice in the old parlour, when I set foot in the hall. She was singing in a low tone. I think I must have lain in her arms, and heard her singing so to me when I was but a baby. The strain was new to me, and yet it was so old that it filled my heart brim-full; like a friend come back from a long absence. (8.34)
David hears Mrs. Copperfield singing to his nameless baby brother and immediately starts remembering how Mrs. Copperfield used to sing to David himself. David's baby brother isn't really an independent figure in the book. He represents David's own vanishing childhood trust and innocence. Once Mrs. Copperfield dies and David is left alone with Mr. Murdstone, it makes narrative sense that this symbol of David's own infancy must also die, to underline David's transition to a new phase of his life.
Thus ended Peggotty's narration. From the moment of my knowing of the death of my mother, the idea of her as she had been of late had vanished from me. I remembered her, from that instant, only as the young mother of my earliest impressions, who had been used to wind her bright curls round and round her finger, and to dance with me at twilight in the parlour. What Peggotty had told me now, was so far from bringing me back to the later period, that it rooted the earlier image in my mind. It may be curious, but it is true. In her death she winged her way back to her calm untroubled youth, and cancelled all the rest. (9.104)
David seems really committed to the idea that youth is "calm" and "untroubled." So, when Mrs. Copperfield passes away, she is at peace again: she becomes like a child winding her "bright curls round and round her finger." And children who are not untroubled grow up too fast – like David himself, or like Emily. What do you think of this line between childhood and adulthood, where childhood is calm and adulthood is troubled? Is childhood really all that calm? Don't David's own experiences provide strong proof that childhood is a time of profound vulnerability and difficulty?
Now, the whole place was, or it should have been, quite as delightful a place as ever; and yet it did not impress me in the same way. I felt rather disappointed with it. (10.69)
When David goes to visit Yarmouth after his mother's death and before he goes to London for the first time, he finds the boat house not "quite [as] delightful a place as ever." It looks different because he is different: David is growing up, and is viewing the world through new eyes. At the same time, David can't perceive his own internal changes. So, he thinks the disappointment of the old, familiar boat house must be some flaw in it rather than in him. This is another example of Dickens using the setting to show a character's transition; the location is a powerful tool of characterization in David Copperfield.