Quote 1
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.
Quote 2
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.
Quote 3
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.