Jeffrey, great comments about the book and movie. I found the book fascinating because it was a mystery: Krakauer knew the end and worked back, wondering how someone like McCandless got to that end. "The Last Season" by Eric Blehm is a similar book, though maybe not as good. The movie was different, about how we need human relationships to survive.
For me, the central point of the movie (which I found very moving) was that "vigorous battle" that Jeffrey mentions above, maybe not with depression, but probably with despair, with the feeling of being lost: in his family relationships, in his suburban upbringing, in finding meaning in his life. It seemed (and maybe only because Sean Penn framed the story this way) that the most important moments in his life was not what happened in Alaska but before, in his relationships with Hal Holbrook's character and Catherine Keener's character and how those connections contrasted so starkly with the connection he had with his parents. To quote another movie: "No man is an island." Come to think of it, maybe "Into the Wild" and "About a Boy" are bookends to the same theme.
Once he got to Alaska, I agree with Buck and others. Reckless with his own life and reckless with the emotions of those who loved him.