This is a modern retelling of the story of the Buddha's enlightenment. The plot structure seems to be modeled after Hesse's Siddhartha, and other legends about the Buddha's enlightenment, in that both Siddhartha and Henry are insulated from suffering by the lavish wealth of their fathers who, in both stories, confuse good parenting with spoiling their sons.
To see the alternate version of this story, where Henry is never challenged to face tragedy and suffering, look at Louisa's account of Franklin's last days, days when Franklin was revealing in subtle ways that he secretly hated poor people and people from other ethnic backgrounds. In other words, the main dilemma of the novel is that this father has set up a stage for his sons where the default is to become morally corrupted and "spoiled" or to do as Henry does, choosing instead to take the suffering of life onto himself voluntarily.
This journey literally takes Henry to the top of a mountain (a common image for religious transcendence or enlightenment), but although he'd really like to do the journey alone, he is not allowed. He comes to see that these details aren't merely coincidences, but rather each companion is there to help him learn something different. From this we learn that although humans have a tendency to become private and alone, actually, other people are necessary in our enlightenment, because they can share points of view that challenge and adjust our fallible points of view.
