Why does McCandless write about himself in third person in his journal [in Decemember 1990 - January 1991]?

This is the point at which McCandless starts to call himself Alex. His use of the third person in his diary almost makes us believe that he truly sees himself as someone else..... like he's an observer of his own life.

The reason McCandless calls himself in the third person is because he isn't who everyone thought he was or what they expect him to be. He is a grown man from "South Dakota" living in the bush going by the name of Alex, he reinvented himself and is someone else; so another way of reinventing he decided to change his perspective and just begin to call himself in the third person instead of the first to let himself know that he is an observer of his own life. He never intended for anyone to read his journal because he never intended to die out in the bush, he claimed that he didn't need help and that he had it all under control.