Chris McCandless' determination is what makes this a compelling story. The struggle he undergoes in order to arrive in Alaska, having made the decisions that he did to abandon society and return to the wild are ones that the vast majority of us have only in our own minds. His determination, that is, allows him to cross over from merely contemplating such a major life change to actually attempting it. I think this is at the root of popular fascination with his story.
A definite drawback to this sense of determination has to be that it sometimes, in fact often, clouded his judgement to the point where his actions and his decisions were either foolish or dangerous, sometimes even hurtful to others. He comes across as a person who sometimes simply plunged ahead with romantic notions without thinking things through clearly. In the end, that impulsiveness (perhaps a better word than determined?) is what costs him his life.