As a writing teacher, I'm forever imploring my students to formulate their thesis statements in a positive way. In other words, think of a thesis statement in terms of a "position statement" or "opinion statement" (as thesis statements are also referred to). The thesis statement must put forth in a clear manner what position you are about to argue. The thesis statement must not be ambiguous. It must be an opinion (your opinion) on the subject that you will support with evidence (details, examples, quotes, etc.)
In the case of Into the Wild, a Wgood thesis statement might look something like, "In the story, Into the Wild, by Jon Krakauer, the main character, Chris McCandless relinquished all ties to his 'world' because he had become disillusioned with his place in society."
And then, you'd spend the rest of your piece supporting that notion.
In an "Author's Note" preceding the tale of Christopher McCandless, who forwent all the opportunities of the "American Dream," forsaking the promise of a lucrative career in exchange for a life of zero material comforts and a constant risk of disease and death by starvation or exposure, Jon Krakauer summed up his protagonist's life as follows:
"Immediately after graduating, with honors, from Emory University in the summer of 1990, McCandless dropped out of sight. He changed his name, gave the entire balance of a twenty-four-thousand-dollar savings account to charity, abandoned his car and most of his possessions, burned all the cash in his wallet. And then he invented a new life for himself, taking up residence at the ragged margin of our society, wandering across North America in search of raw, transcendent experience."
When formulating a thesis statement for Into the Wild, then, one would logically begin with that observation regarding the decisions McCandless made that shaped and prematurely ended his life. Chris McCandless, for reasons ultimately known only to him, was dissatisfied with the way his life was evolving within the conventional confines of society. He eschewed ambition and materialism in favor of living as close to nature and as far removed form society as he possibly could. He was alienated from his parents, who loved him, but who could never truly understand him. A thesis statement, therefore, could read as follows: "Christopher McCandless rejected the American Dream as traditionally defined and sought a more emotionally fulfilling existence in the wilds of Alaska. His demise from starvation or accidental poisoning was the result of underestimating the scale of distinction between "civilization" and the uncivilized wilds into which he retreated."