Quote 16
The word still rings in the air: Swine! Never has he felt such an elemental rage. He would like to give the boy what he deserves: a sound thrashing. Phrases that all his life he has avoided seem suddenly just and right: Teach him a lesson, Show him his place. So this is what it is like, he thinks! This is what it is like to be a savage! (23.5)
Here, we see pure violence and hatred raging through David's entire being – he even thinks of himself as a "savage." All bets and rules are off; as far as David is concerned, it's go time.
Quote 17
"After a certain age, all affairs are serious. Like heart attacks." (5.62)
Here, David responds to his lawyer's question about whether or not his relationship is serious. Nice play on the heart imagery here, David. You can see love as a heart attack, since the heart is supposedly the seat of desire and emotion, but heart attacks are also not uncommon afflictions among the aging and elderly.
Quote 18
He sighs again. How brief the summer, before the autumn and then the winter! (10.62)
Here, David ever-so-poetically bemoans the fact that his youth is over and that middle and old age are creeping up on him.