In Into the Wild, why did Chris McCandless keep to himself?

Without being able to directly ask Chris McCandless, the answer to this question will probably forever remain a mystery.  Krakauer does a nice job of trying to "get inside" McCandless and figure out what makes him tick, but it's still Krakauer's best guess.  Krakauer also does a nice job of illustrating how McCandless wasn't the only ascetic to be drawn to the road and Alaska.  Krakauer spends chapters eight and nine comparing McCandless to Gene Rosselini,...

Without being able to directly ask Chris McCandless, the answer to this question will probably forever remain a mystery.  Krakauer does a nice job of trying to "get inside" McCandless and figure out what makes him tick, but it's still Krakauer's best guess.  Krakauer also does a nice job of illustrating how McCandless wasn't the only ascetic to be drawn to the road and Alaska.  Krakauer spends chapters eight and nine comparing McCandless to Gene Rosselini, John Waterman, Carl McCunn, and Everett Reuss.  All of those men felt the pull of a solitary life.  They all died in that life too.  That doesn't mean that they were unsuccessful though.  Similarly, I don't think that McCandless failed in his quest either.  

But why exactly McCandless wanted to live the life that he chose to live might best be explained by using a Henry David Thoreau quote.  

"I wanted to live deep and suck out all the marrow of life, to live so sturdily and Spartan-like as to put to rout all that was not life, to cut a broad swath and shave close, to drive life into a corner, and reduce it to its lowest terms."

I think that is exactly why McCandless kept to himself.  He wanted to live his life to the fullest.  For McCandless that meant paring his life down to the bare essentials.  It also meant that he needed to be free to make his own decisions about his own life direction all of the time.  In order to do that, McCandless had to live a solitary life.  Being around people always resulted in those people trying to force their will and opinion on McCandless.  Even Ronald Franz tried to do that to McCandless at one point. 

After attending church that Sunday, Franz decided to talk to Alex “about how he was living. Somebody needed to convince him to get an education and a job and make something of his life.”

By living alone, McCandless is free to do what he wants to do.