how was a relationship forged between hans hubermann and erik vandenburgs son max? why did hans feel compelled to help max?

Hans befriends a German Jew named Erik Vandenburg, who teaches Hans to play the accordion. The day the platoom is to go into battle, the Sergeant asks which one of them has good handwriting. None volunteered. The Sergeant says that whichever man does will not be going into battle, yet no man wants to seem like a coward. Erik nominates Hans, who is sent to write letters for the captain. The rest of the men are all killed. Hans credits Erik with saving his life. Hans is surprised to find that Erik has a young son named Max. Hans leaves Erik's widow with his name and address and offers to help if they should need anything. With the Jewish crackdown by the Nazis, Max's mother gives Max a piece of paper with Hans Hubermann's name and address. Hans is compelled to help the son of the man who saved his life.

http://www.gradesaver.com/the-book-thief/study-guide/section5/