Raleigh and Jack are BFFs. Raleigh admires Jack Fawcett because everyone admires Jack Fawcett. Jack admires Raleigh because "He is absolutely honest and decent in every sense of the word and we know each other inside and out" (1.18). Because of this mutual admiration, Raleigh joins Jack's expedition to the Amazon.
Raleigh is honest about how much he hates being in the jungle. He gets sick, tired, and anxious as any normal person being devoured by parasites would. This prompts Jack to do his best impression of his father and turn against Raleigh for being a weakling.
Poor Raleigh feels like he's not only losing himself and his sanity in the jungle, but he's losing his friend. He writes in a letter, "I feel 'awful lonesome' for real friendship" (20.66). But like a loyal dog, Raleigh follows Jack all the way up until they disappear together.
Man's best friend deserves better treatment.