Quote 10
NASA, however, is absolutely shitting itself [...] To them, equipment failure is terrifying. To me, it's "Tuesday." (13.57)
What a difference a few months make. Do you remember how frightened Mark was on Day 1 of this ordeal? Look at him now! This passage shows us that courage isn't about not feeling fear—it's about learning to manage it.
Quote 11
The worst moments in life are heralded by small observations. The tiny lump on your side that wasn't there before. Coming home [...] and seeing two wineglasses in the sink. (17.97)
Although Mark has gotten used to living in fear, he hasn't exactly transformed into Braveheart. The fact of the matter is that something could go disastrously wrong in an instant, and Mark needs to be vigilant to prevent that from happening. While good, that sort of attention is going to drive you crazy at some point.
Quote 12
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.