Because Joseph never gives up on trying to see his daughter.
I think a theme in this book is too never give up.
Jack is a compasionate character that cares about his friends, we have proof of this, because he helped Joseph in the fight.
An example of figurative language is when Jack said that he couldn't see Jupiter, so neither could Joseph. I think it meant that Joseph couldn't see the planet Jupiter, which means that he also couldn't see his daughter Jupiter.
Joseph is a very round character with a lot of development. Becasue in the beggining he was very paranoid and cautious. But later in the story he's friendly, and even self-sacrificing.
I liked the book, but I thought that it was a bit anticlimactic. Because Joseph never really got to see his daughter, only a picture. I also don't like that he died in the end, I felt like it was kid of forced.