In chapter 16 Ms. Nesbitt dies. In chapter 17 the family starts eating only one meal per day. In chapter 18 It's Christmas and the family gives gifts to each other. In chapter 19 Miranda finds out Peter died and saves the family from smoke filling up the house . In chapter 20 they start getting electricity. In chapter 21 the Mayor gives them lots of food.
The theme in Life as We Knew It is that things are going to get worse before they get better. In the first few chapters Things are almost normal. In chapters 6-10 things get a lot worse. In chapters 11-15 things are at their absolute worst. But in chapters 16-21 things start getting better . The book is saying things will get worse but there is still light at the end of the tunnel.
The one thing keeping the characters motivated is the hope of things going back to normal. the only thing the characters want is the life they had before. The characters don't want to make the world perfect, they just want it back to normal. They just want to have 3 meals,heat and electricity. They just want life to go back to how they knew it.