This photo always makes me remember that feeling. The feeling of going on an adventure with my sister, just the two of us, the world expanding like an open book.
-- Stella (chapter 1)

Importance: Stella's hospital room is filled with reminders of her sister, Abby, who has recently died (though the reader does not yet know this). In the photograph she is looking at, she and Abby had been visiting the nearby park to look at the holiday lights. This is foreshadowing for the end of the novel when Stella will make the decision to seize the day and take Will to see these same lights.

When you have CF, you sort of get used to the idea of dying young. No, I'm terrified for my parents. And what will become of them if the worst does happen, now that they don't have each other.
-- Stella (chapter 1)

Importance: Stella's parents have recently divorced, and also recently suffered...