you can explain this using the themes
The theme of the duality of human nature. People are capable of both good and evil, the dark and the light. Death hopes that people gravitate to the light but from the war this looks doubtful.
In The Book Thief Death says repeatedly that this is only the story of a girl. But at its deepest level, do you feel that one of the key themes of the story is the way that human love and kindness, however fleeting, have a kind of permanent power over death?
Zusak's answer;
"I concentrated mostly on the idea that humans have great beauty and great ‘ugly’ (as Death puts it) in them, and we all struggle to steer ourselves, hopefully, towards the beauty. I guess the old saying is true – that death is what makes life worthwhile. Knowing that we are not here forever makes us appreciate things, and it’s often those fleeting kindnesses and loves that define us in the end."
Love
The Book Thief focuses on characters who are learning to love in the face of great hatred. There is also romantic love in the novel. Of course, it's an innocent childlike romance that tragedy cuts short. The novel is set in a Nazi Germany, where giving another person something as seemingly small as a crust of stale bread or even a smile could be seen as act of immense kindness.
These acts almost always involve penalty. To love in such a harsh world is both a necessity and a triumph for the characters of The Book Thief.
Courage
The characters in The Book Thief exhibit great courage in the face of great adversity. It takes courage for the Hubermanns to take in Max Vandenburg, a young Jewish man. It takes extreme courage for Max to make the trip to their home in the first place, and even to trust them. As the characters grow and change, courage becomes a bigger and bigger factor in their lives. In fact, it becomes a life-sustaining attribute and a testament to their positive humanity.
The courage to resist unjust laws and practices and to display their resistance publically, even in small ways, makes these characters an inspiration to many readers.
There is also the theme of kindness and the way the characters treat and feel about one another.
