What is the significance of nightmares in the book thief?

There is a communion of sorts between Max and Liesel: a metaphysical sharing of thought. Max and Liesel both have nightmares, and one night Liesel asks Max about them. Max tells her he sees himself waving goodbye to his family, and Liesel tells him about her brother. Liesel brings Max a newspaper she finds in a garbage can, and Max gratefully does the crossword. On Liesel's birthday, Hans and Rosa give her The Mud Men, a book about a "strange father and son," and Max apologizes for not getting her anything. Liesel graciously hugs Max for the first time, and Max wonders what he could do for her.