Why is Hans Junior is the most naive person in The Book Thief?
As Chapter 2 of this book clearly shows, which is the last time when Hans Junior meets with his father, Hans Junior is incredibly naive because of the way in which he has swallowed Hitler's ideas and ideology completely, without any questioning or criticism. This of course differs incredibly from his father, who is unable to join the Nazi party because of his opposition to Hitler and his ideas and his inability to do and say what is necessary in order to be considered "safe." Note how, when Hans Junior sees Liesl reading a book, he says she should be reading Mein Kampf rather than any other "trash." However, it is in his dialogue that his true naivety is revealed, when he attacks his father for his opinions:
You're either for the Fuhrer or against him--and I can see that you're against him. You always have been... It's pathetic--how a man can stand by and do nothing as a whole nation cleans out the garbage and makes itself great.
Note how Hans Junior describes the Nazi project, as "cleaning out the garbage." It is clear, unthinking propaganda, and the reference to how Germany through this process is making itself "great" demonstrates the significant extent of Hans Junior's naivety. He is unable to question or criticise any aspect of Nazi ideology, and it is this inability that results in him calling his father a "coward" and his estrangement from his family.