what was Chris McCandless running away from

There are a lot of layers to this. Certainly Chris is a counter-culture rebel. He abandons the capitalist system of consumerism. He wants to, as Thoreau states, Live deliberately, and suck the marrow out of life. Chris also has father issues which he seeks to sever himself of. I think there is also an element that points to Chris running away from himself. Chris really doesn't know exactly why he feels the way he does. He goes looking for answers instead of looking within. Chris's idealism clouds his judgment.