What are the main themes in The Book Thief, and why?

There are several important themes in The Book Thief. One of the main themes is the theme of survival. This is ironic, as the novel is narrated by the Death itself. The Death reflects throughout the novel that some people, like Hans Hubermann and Max, manage to cheat death several times and survive, while some people die purely because they end up in the wrong place in the wrong time. The Death compares accidental survival (that of Liesel surviving the bombing because she happened to be in the basement) to the survival that required enormous strength and sacrifice (that of Max, a Jew in wartime Germany.)
The Death also invokes the theme of fate. Rudy's parents refused to send him away to serve in the youth section of Nazi party, but the Death notes that had they done that, then "may be, may be he would survive." Instead, Rudy died in the bombing of Himmel street. His father was drafted into the Army as a consequence of not cooperating with authorities, which was a certain death sentence—but he survived.
It is important to mention the related themes of honor and morality, which are demonstrated by Hans and Rosa hiding Max in their home, at the great personal risk. Finally, perseverance is another theme explored in the novel. Liesel had difficulty learning how to read, but she (and Hans) did not give up until she became a proficient reader inventing creative ways to obtain reading material.