Quote 13
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 14
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.
Quote 15
In four hours, I'm going to ride a giant explosion into orbit. This is something I've done a few times before, but never with a jury-rigged mess like this. (25.187)
Given that Mark is about to be launched into space with only a strip of Hab canvas separating him from the lifeless void, we'd say he has good reason to be afraid. The bright side is that this will be the last time he has to do something like this—hopefully.