Jon Krakauer structures the first two chapters, and most of the book, in an interesting way so as to affect his readers. The description in the book - in other words, the non-dialogue - is written in long, winding, descriptive sentences. The dialogue of McCandless and those who encountered or knew him is written in short, simple sentences; sometimes the dialogue is not even an entire sentence.
There are a couple of reasons Krakauer may have chosen to structure his writing in this way. The first is to parallel McCandless' personality and journey. For example, when anyone talks, he is short-winded and to the point:
"...Gallien remembers. "He was determined. Real gung ho. The word that comes to mind is excited." (p. 6)
This parallels McCandless' personality and his plans for going out into the wilderness. He had not truly thought through how he was going live, eat, get there, survive, or leave. He had, simply, wild, unconnected, underdeveloped, unprepared plans that he used to get to Alaska. His thoughts are much like the way that everyone speaks in the book.
However, Krakauer writes the commentary as long, sweeping, detailed sentences.
"Gallien, a union electrician, was on his way to Anchorage, 240 miles beyond Denali on the George Parks Highway; he told Alex he'd drop him off wherever he wanted. Alex's backpack looked as though t weighed only twenty-five or thirty pounds, which struck Gallien - an accomplished hunter and woodsman - as an improbably light load for a stay of several months in the back country, especially so early in the spring (p.4)."
These sentences parallel the thought that McCandless should have given his plans. Krakauer's commentary, with its long-windedness, leads the reader to believe that if McCandless had just thought out his problems, been older and more experienced, he would have survived this experience and it may have been the transcendentalism he desired.
The structure of the sentences also succeeds in drawing the reader into the story when considered in conjunction with the content of the first 2 chapters. The first chapter introduces McCandless as he begins his journey into Alaska (not his entire journey, mind you, simply his entrance into Alaska). In one short chapter, that is covered. In the second chapter, his death is covered. Therefore, in 2 short chapter, the author covers his life and death. But the detailed sentences hint at something more. They draw the reader in and make them think there must be more to this story. If these sentences were short and simple, the reader may wonder why there is an entire book when it sounds like it could be covered in 2 chapters. So the long-winded sentences make the reader want to read and know more and understand why McCandless was as naive as he appeared.