Both Liesel and MaX were impacted dramatically by the war--they both had families taken away, destroyed or killed as a result of it. For Liesel, it was because her father supposedly had communist leanings; for Max, it was because he and his family were Jews. So, they lose, entirely, their real families. Also, they gain an adopted family through the Hubermanns. They find shelter, refuge, comfort and security in the Hubermann household. Liesel and Max also have guilt issues and nightmares regarding their role in or presence during the final moments of their family. Max feels guilty for not trying to stop the SS from taking his family; Liesel feels awful for her brother's death. Liesel and Max both love reading and books, and bond quite a bit over that. They are both young, and survivors of the war, when everyone else that they love is killed or disappears.
Max and Liesel first form a bond over nightmares; they awake one night with nightmares about their families, and form a connection in that way. Books then also help them to form more bonds. Max gives Liesel someone to help, someone to be a friend to, someone to help LIVE in a war where everyone had died. She helps him to feel like he has family again, and she feels the same way with him. As difficulties increase, and Max must leave, he gives Liesel a reason to rebel, to hope, to wish for the war to be over. He also gives her a reason to survive, and to write about their story. His book to her inspires her to write down her own events, and that, in the end, is what saves her. I hope that those thoughts helped; good luck!