I think this question can most succinctly be answered by refering to Chapter Six, which details the relationship that Chris McCandless formed with Ron Franz. What is notable about this relationship is the profound way in which it impacted Ron Franz, and how as a result he left the security of his life and did what Chris recommended he did, and lived out in the wilderness by himself. In addition, Ron Franz also asked Chris if he could adopt him as his grandson. This is key because Chris's silence about this topic and the way that he refuses to give him a negative, but also the way that he leaves Ron's life, indicates the way that the relationship with his family has made him a commitment-phobe. Note what the author says regarding this aspect of Chris and how happy he was to be finally heading for Alaska:
McCandless was thrilled to be on his way north, and he was relieved as well--relieved that he had again evaded the impending threat of human intimacy, of friendship, and all the messy emotional baggage that comes with it. He had fled the claustrophobic confines of his family. He'd successfully kept Jan Burres and Wayne Westerberg at arm's length, flitting out of their lives beofre anything was expected of him. And now he'd slipped painlessly out of Ron Franz's life as well.
In addition, note what Chris writes to Ron in the final letter Ron receives from Chris:
You are wrong if you think Joy emanates only or principally from human relationships... My point is that you do not need me or anyone else around to bring this new kind of light into your life.
Clearly, for Chris, attachments and relationships were the kind of things that he had rejected, and we can perhaps infer that this decision to spurn human closeness came from his initial decision to walk away from his family.