Chapter Six is the section of this novel that deals with the relationship between Franz and Chris McCandless. You would do well to re-read it and pay particular attention to how Ronald Franz's account adds to our understanding of Chris and what drove him. In particular, while Franz is obviously impressed by Chris's intelligence and ability to survive, it is also interesting to note the unresolved anger that is so much a big part of Chris's character. Note the following quote:
Not infrequently during their visits, Franz recalls, McCandless's face would darken with anger and he'd fulminate about his parents or politicians or the endemic idiocy of mainstream American life. Worried about alienating the boy, Franz said little during such outbursts and let him rant.
It is clear that so much of what drove Chris to do what he did was his very real sense of frustration and anger with his parents and American life as a whole. He chose to reject mainstream life largely because of this.
Another factor that is made explicit in this chapter is the way that Chris McCandless deliberately avoided any kind of relationship that would ensnare him and keep him back from doing what he wanted to do. Note the comment that Krakauer makes after Alex gives a non-commital response to Franz's offer to adopt Alex:
McCandless was thrilled to be on his way north, and he was relieved as well--relieved that he had again evaded the impending threat of human intimacy, of friendship, and all the messy emotional baggage that comes with it.
This is of course Krakauer's own opinion, but it clearly indicates the way in which Chris McCandless tried to keep himself aloof from human bonds and relationships. These are the two most important elements that we learn about Chris's character from his relationship with Ronald Franz.