He whacked Mark on the side of the head—the closest thing to affection the soldier ever showed. (1.8)
Alec isn't great at showing affection… but he is great at displays of chummy violence. Eh. Sometimes you take what you can get?
Mark still couldn't tell if he was laughing at the subject matter or just at the sounds coming out of the Toad. Either way, such moments were few and far between, and it felt good to laugh, as it did to see Trina's face light up. (2.38)
The friendship between everyone who survived the sun-flares is tremendous; after all, they only have each other. So it's pretty devastating when half of them die in the first few chapters.
But how could he have known she'd turn into this beautiful thing with the hot body and the dazzling eyes? (8.3)
Pshh, don't you just hate it when that happens?
But mostly he can tell that she sees them as lifelong buds, nothing more. That the thought of kissing in his basement is just plain silly. (8.18)
Aww, rejection-city. Deal with it Marky-boy.
Everything in him wanted to do the opposite of what she'd just ordered. He wanted to help his friend. (13.2)
Mark's instinct to save Misty goes against the reality of the situation—by saving her, he'd be dooming himself. He's powerless to help his friend, which is utterly devastating.
Their lives were as linked together as the rocks and earth of the mountain they'd just slept upon. (15.2)
This quote is talking about Alec and Mark; they might not be anything alike, but they save each other time after time, and they're like father and son by the end of the book.
It had only taken ten minutes of her being missing for the most sinking feeling of helplessness to hit him. (28.1)
Mark and Trina's friendship is so strong that Mark feels helpless after Trina's been missing for ten minutes. Yeesh. Talk about separation anxiety.
"It doesn't mean you… have it. Humans do crazy things to survive. Have you not spent the last year seeing that every day?" (45.42)
Friends cheer other friends, up. It's what they do. Even when it has to do with a brain-eating disease that's most likely taken root in your friend's head.
"You know… what to do… when I'm done," he said to Alec, words a struggle now. (67.13)
The final act of friendship between Alec and Mark: Alec has to kill everyone to save them from suffering from the Flare. Ouch.
But he still heard what she whispered into his ear before the Berg came crashing into the building. "Mark." (67.33)
At the very, very, very end of the book, we're actually put at ease because Trina remembers Mark. By restoring their friendship in this last instance, their deaths really don't seem as sad as they could've been.