How we cite our quotes: (Chapter.Paragraph)
Quote #7
I've been in mortal danger for months; I'm kind of used to it now. But I'm nervous again. Dying would suck, but my crewmates dying would be way worse. (18.125)
It's one thing to die, but it's whole different thing to lead five of your closest friends to their death. Mark has long-since made peace with his own possible death: in fact, he never really expected to make it this far. Bringing in other people only raises the stakes higher.
Quote #8
I'm totally exhausted but I can't sleep. Every sound scares the s*** out of me. Is that the Hab popping? No. Okay...What was that!? (20.90)
At this point, fear is pretty much a Pavlovian Response for Mark. The guy has experienced so much crazy stuff since getting stuck on Mars that even the smallest thing gets transformed into a crazy disaster in his mind. He can't let his fear trip him up, however—he needs to keep his eyes on the prize.
Quote #9
I never realized how utterly silent Mars is. It's a desert world with practically no atmosphere to convey sound. I could hear my own heartbeat. (21.167)
Mark's fears are amplified by the fact that he's completely and utterly alone. As every horror movie ever has shown us, isolation has a way of stoking the fires of fear. Of course, Mark has a pretty low chance of running into a deranged serial killer on Mars, so maybe it's a net positive.