The Doctor Zhivago quote is a definite standout when compared to most of the other quotes that Krakauer starts each chapter with. The previous quotes that begin the other chapters are very much focused on man being an individual and being alone. If the quote doesn't focus on the individual, then it focuses on being out in the wilderness. This is why Thoreau is used throughout the text. His writings embody the idea that spiritual wholeness is found through a solitary, spiritual connection with the wilderness.
The Doctor Zhivago quote is nearly the antithesis of this idea. The quote does talk about spirituality, but the spiritual core that it talks about is found through a person's connection with other people. If a reader is paying close attention to the text, then that emphasis on loving one's neighbor should alert readers to a coming shift, and that shift is in how McCandless views happiness. McCandless went out into the Alaskan bush in order to experience the solitary glory that he so rabidly chases after. His entire goal was to be alone, and the self-inflicted solitary environment allowed him to learn just how much he appreciated people and companionship.
Chris might never have become a social butterfly, but he did learn as indicated by his journal that happiness is best and most "real" when shared. This echoes the Zhivago quote because it is impossible to love one's neighbor if no neighbors exist. His happiness is not obtainable or real when alone. McCandless learned a slight variation of the idea that people can't know happiness without experiencing sadness. He can't have happiness if there is not one to share it with.
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.
The quote from Doctor Zhivago that opens Chapter 18 was highlighted by Chris and is relevant on a number of levels. First and most importantly, it spoke to Chris's need to spend time alone to discover a personal faith and spiritual awakening; although this can be accomplished in many ways, Chris viewed isolation as his personal method. It also mentions "life as sacrifice," an ideal that Chris shared with Tolstoy and Thoreau, among others. Finally, it also speaks to Chris's goal after achieving his personal growth:
To begin with, love of one's neighbor, which is the supreme form of vital energy. Once it fills the heart of man it has to overflow and spend itself.
(Krakauer, Into the Wild, from Doctor Zhivago: Amazon.com)
The bolded line above was underlined by Chris; it can be assumed that he felt his connection with society and others was dependent on his own personal growth, and once he learned to "love one's neighbor" he could find it possible to return to civilization and "spend" his love. This reading is echoed later in the chapter by Chris when he wrote: "HAPPINESS IS ONLY REAL WHEN SHARED [sic]." This seems to be in opposition to his normal ideals, and Krakauer wonders if this was the epiphany that Chris had been searching for all along. If so, his decision to return to society was likely directly related to his epiphanies.