You are my very favorite person on the planet,’ he murmurs in my ear, and I smile. These are the words we’ve chosen instead of the more standard I love you, and to me they mean so much more.
-- Will (Chapter 1)

Importance: These words that Will and Iris say to one another instead of the typical "I love you" alert Iris that it is not Will who is sending her the texts that she had previously believed were from him. The writer of the texts tells Iris that he loves her, not that she is his favorite person.

My education has taught me to see nature and nurture as equal propositions, but my job has taught me nurture wins out every time. Especially when it’s lacking. The more messed up the parents, the more messed up the kid. It’s really that simple.
-- Narrator (Chapter 2)

Importance: Iris believes as a result of her experience...