Home, sweet home.
After nearly two years of struggle, Mark Watney has finally escaped Mars. That's a pretty impressive feat, especially given that the book starts with the declaration that he will likely be "the only human being to have died on Mars" (1.6).
He didn't do it on his own, however. NASA helped. The China National Space Administration helped. Most importantly perhaps, the Hermes crew helped, risking their lives to bring their friend home.
Because of this, it's only fitting that the novel ends with an ode to the human spirit. Mark knows that he only survived "because every human being has a basic instinct to help each other" even if "it might not seem that way sometimes" (26.424).
Everyone could've saved a ton of time, stress, and money if they had simply gone about their lives without paying Mark any mind. Instead, countless people (most of whom have never even met Mark) put their lives and well-being at risk to bring one man home. Basically, all we're saying is give peace a chance, y'all.