Chris McCandless is emotionally prepared when he sets out into the Alaskan Wild, but he is such a romantic when it comes to his beliefs about the wild that he is not prepared in the domain of what he needs to survive.
Chris goes into the wild bringing a rifle, some ammunition, rice, a small cook stove, books, a book about plants, and a few other items. He leaves his map behind on the truck dashboard of the man who drops him off.
Chris was a highly intelligent young man who believed that he could live off the wild. This might have been true except he did not know that the river would flood during the time he was there. When he was ready to go back to civilization the river was way too high and dangerous to cross. Without a map he could not know where he could safely cross. He just made some stupid choices but I don't think he was a kook. He was a product of the anti-establishment era who felt that he had a destiny to fullfill.