I don't even know who'll read this. I guess someone will find it eventually. Maybe a hundred years from now. (1.5)
At this point, Mark doesn't think anyone will even realize that he survived, much less rescue him. This creates a sense of isolation that's both figurative and literal: he's afraid of not only being abandoned but forgotten. Luckily, this pessimistic perspective is quickly squashed by his seemingly limitless supply of optimism.
Mars is a barren wasteland and I am completely alone here. I already knew that, of course. But there's a difference between knowing it and really experiencing it. (7.101)
Whenever Mark goes out on the surface, he gets a brutal reminder of how alone he truly is. The Hab provides a certain degree of comfort—although he still can't communicate with Earth, the mere presence of this man-made building reminds him that there's a home waiting for him. Unsurprisingly, the barren landscape of Mars has the opposite effect.
"Communication would be a great thing. But traversing [...] to Ares 4 is incredibly dangerous. [...] If we could talk to him, we'd certainly tell him that." (8.19)
If NASA could communicate with Mark right now, they'd tell him that communication isn't as important as survival. Luckily for everyone involved, however, Mark isn't that crazy—yet. Although he's not ready to throw caution to the wind and travel to the Ares 4 site, he really needs someone to talk to.
"If he's lost hope, he won't care about survival. His only concern will be making it to the radio." (8.147)
For many, prolonged isolation can be a one-way ticket to crazy-town. Mark never goes bonkers (luckily, they didn't bring any volleyballs along for the trip), but the possibility is a very real one.
Jesus Christ, I'd give anything for a five-minute conversation with anyone. Anyone, anywhere. About anything. (9.42-43)
By now, this whole isolation thing is getting to Mark's head. Although he's dealing with life-or-death situations on a constant basis, his biggest concern is hanging out with people. Anyone can get used to living in a dangerous environment, but there isn't a human alive who can get used to being alone.
This was an insane plan and somehow it worked! I'm going to be talking to someone again. I spent three months as the loneliest man in history and it's finally over. (11.23)
Getting in contact in NASA is a big deal—the only thing bigger would be if Mark stumbled across a fully-fueled, state-of-the-art Martian spaceship. His situation is still as terrifying as ever—and his odds for survival are still catastrophically low—but now he has reason to hope. Hope can be a powerful thing.
Now that NASA can talk to me, they won't shut the hell up. (13.4)
How quickly things change! We're sure that Mark wouldn't go back to radio silence if you paid him, but we can understand why he reacts like this—he simply misses his independence.
Pathfinder's dead. I've lost my ability to contact Earth. I'm on my own. (17.136-137)
Here's a trick for reading The Martian—if things are looking up, then you're only pages away from it all tumbling back down. You wanted independence, Mark? Well, your wish has been granted.
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)
With the Hab disassembled, Mark feels the full weight of his isolation. Although he knows that he's beginning the final leg of his journey (whether he survives or not), he still can't help but feel unsettled. To be honest, we'd suggest throwing some disco on the stereo and blaring it to high heavens. Rovers have good sound systems, right?
"Twelve days," Cathy said to the camera. "All of Earth is watching but powerless to help." (22.25)
Though we've focused a lot on Mark in this section, it's worth mentioning how the people of Earth feel about this ordeal. They feel isolated from Mark too, but their dissatisfaction is rooted in their feelings of powerlessness to help.