Maybe he thought because he loved the wilderness so much that the wilderness would love him back and not kill him. He knew there was a chance he could die, but he didn't think it would really happen to him.
The book Chris bought that told about wild plants he could eat didn't say anything about the wild potato seeds being poison. Chris had been eating fairly well up to that point. He hunted everyday and wrote down the small animals he shot and cooked for food. He picked berries and other plants he found in the woods to eat. He had been eating wild potatoes for several weeks, but towards the end of summer the roots of wild potatoes get tough and stringy. They probably didn't taste good anymore. He was hungry. He had a large stash of seeds and pods, more than enough to plant, and so he decided to eat them. It wasn't much different from eating sunflower seeds. He had no idea they were poison! How could he know? And it made sense to think they were edible since the rest of the plant was edible.
Chris didn't die from an "extreme" act. It was a mistake. Of course, he wouldn't have died if he hadn't tried to live alone in the wilderness. Like mountain climbers wouldn't freeze to death or fall or get caught in an avalanche if they hadn't chosen to climb the mountain in the first place. Soldiers wouldn't get killed in Iraq if they hadn't decided to go in the military. The point is that Chris McCandless could not be happy until he lived out his dream of a journey into Alaska. He refused to see the risk. "He didn't think the odds applied to him. We were always trying to pull him back from the edge" his father said (p. 109). Chris's friend Andy Horowitz said Chris always wanted more of life and was never satisfied. He was curious and hungry for experiences. Andy said, "He was looking for more adventure and freedom than today's society gives people" (p. 174). Chris's dream was for adventure. He wanted to be free, live off the land, and be completely self-reliant.
Living extremely was a spiritual quest. Christ saw his time alone in the wilderness as a way to grow spiritually, to find himself, and know what he wanted to do with the rest of his life. He read books by people like Emerson and Thoreau and took them very seriously. He read them over and underlined them and made notes in the margins. It was like he had a cause he believed in, and he wanted to be obedient to it. For example, he took as little as possible with him into the bush and refused to fly to Alaska because he thought that would be cheating. He wanted to do everything right so he could get the most out of the experience. He took books with him into the wild instead of equipment, food and supplies! (He left behind his long underwear and the warm clothing his friend Burres had given him.) This shows he thought of it as a spiritual quest more than a physical challenge. The whole point was to gain meaning and be transformed. He called his trip the "great Alaskan odyssey" (Krakauer 45). Webster defines the word odyssey as "an intellectual or spiritual wandering or quest."
At the beginning of Chapter 16 the author put a quotation by Estwick Evans who was talking about his own journey into the wilderness: "I wished to acquire the simplicity, native feelings, and virtues of savage life; to divest myself of the factitious habits, prejudices and imperfections of civilization; and to find, admidst the solitude and grandeur of the western wilds, more correct views of human nature and the true interests of man. The season of snows was preferred, that I might experience the pleasure of suffering, and the novelty of danger" (Krakauer 157). If Chris McCandless, or any other adventurer, were explaining his reasons for living an extreme life, these could be them exactly.
Although some people thought Chris was arrogant, many people thought he was a hero. They admired what he tried to do. He had a dream and he lived it. It wasn't really his fault that it turned out so badly. He was trying to get meaning out of life. As Dr. Frankl wrote, "Ultimately, man should not ask what the meaning of his life is, but rather must recognize that it is he who is asked. In a word, each man is questioned by life; and he can only answer to life by answering for his own life; to life he can only respond by being responsible" (Frankl 172).
Answers.com web site: self-esteem. http://www.answers.com/self-esteem