Chapter 18 is the last chapter, and in that chapter, Krakauer unravels the tragic truth of Chris McCandless’s death. For most of the book, Chris was running from his old life, from superficial relationships and a material world. He was seeking profound transcendental ideals and a life alone where he could experience and live life to the fullest. However, we learn in this chapter that it isn’t until right before his death that he comes to terms with the pain of his past and thinks to go and make amends with his family.
The Doctor Zhivago quote relates to the revelation that Chris has during reading. He figures out that he wants to share his life with other people. While that might have meant he was ready to make up with his family, it probably also meant spending time with his friends and others who he had met on his journey. For so much of the novel Chris eschews companionship and deep personal relationships—but through his reading of Doctor Zhivago and his time in the Alaskan wilderness, he learns that the only life worth living is one that is shared.
In the chapter, he relates that "happiness is only real when shared."
The revelation comes from Doctor Zhivago and can help us understand the significance of the epigraph at the start of the chapter. The epigraph passage shows us that Chris is beginning to expand his idea of the point of life. Not only is life supposed to be shared with others, but self-sacrifice is essential to a life well lived. Chris wanted to take what he has gained, the fruit of his journey, and share it with others. He figures out that it is impossible to live a life of sacrifice without others around him to love.
This revelation comes at the worst time because Chris dies before he is ever able to realize the truth of these statements. He dies before he is ever able to share what he has gained from his time away. The quote does help explain why Chris felt it was time to leave Alaska, as he wanted to go and spend time with others.