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.