Why does chapter 6 end with a discussion on the meaning of sorrow? What is the meaning of sorrow according to Signora Bang?

Having never understood the concept of sorrow, David often lives in a state of hopelessness..... and yet he keeps going. Sophie sees sorrow as something that shapes one's life, and yet, people need to go on and be as happy as they can be in the face of unhappiness. She tells David that sorrow doesn't last forever.

" All suffering has an end, David, if only you wait long enough ... sorrow has its life just like people. Sorrow is born and lives and dies. And when it's dead and gone, someone's left behind to remember it. Exactly like people."