Quote 1
"It's enough to distract me," cried my mother. "In my honeymoon, too, when my most inveterate enemy might relent, one would think, and not envy me a little peace of mind and happiness. Davy, you naughty boy! Peggotty, you savage creature! Oh, dear me!" cried my mother, turning from one of us to the other, in her pettish wilful manner, "what a troublesome world this is, when one has the most right to expect it to be as agreeable as possible!" (4.10)
When David comes home to find Mrs. Copperfield married to Mr. Murdstone, he's not exactly overcome with joy. And in her disappointment at David's reaction, Mrs. Copperfield strikes out at David and Peggotty for being "naughty" and "savage" at not being happy for her. Mrs. Copperfield can't understand why there is still suffering in a world when "one has the most right to expect it to be as agreeable as possible." The thing is, why do we have a right to expect the world to be nice? Not to sound like our crotchety grandparents or anything, but who says life is fair? Maybe Mrs. Copperfield's assumption that she's owed a good life makes her all the more disappointed and unhappy when she suffers.
Quote 2
"How can you be so aggravating," said my mother, shedding more tears than before, "as to talk in such an unjust manner! How can you go on as if it was all settled and arranged, Peggotty, when I tell you over and over again, you cruel thing, that beyond the commonest civilities nothing has passed! You talk of admiration. What am I to do? If people are so silly as to indulge the sentiment, is it my fault?" (2.54)
Mrs. Copperfield immediately gets irritated when she thinks that Peggotty disapproves of Mr. Murdstone's admiration of her. Whenever Mrs. Copperfield hears anything that she interprets as criticism, she lashes out like a spoiled child. Her selfish nature allows her to put her own self-interest above her family's, with disastrous results for Mrs. Copperfield herself.
Quote 3
I hand the first book to my mother. Perhaps it is a grammar, perhaps a history, or geography. I take a last drowning look at the page as I give it into her hand, and start off aloud at a racing pace while I have got it fresh. I trip over a word. Mr. Murdstone looks up. I trip over another word. Miss Murdstone looks up. I redden, tumble over half-a-dozen words, and stop. I think my mother would show me the book if she dared, but she does not dare. (4.74)
Before David falls into the hands of the Murdstones, he shows a love of reading – remember Peggotty's crocodile book! But once the Murdstones are watching him like hawks while he recites his lessons, just waiting for him to mess up, suddenly all of David's smarts dry up. This is the most basic lesson of this book: treat a kid cruelly, and you'll get nothing out of him. Treat him kindly, and you'll get a happy and productive kid.