Every time I watch this scene I can’t help but think that Bellamy wanted to say something yet couldn’t…
You know, think about it: They have just gone through hell; killed a lot of innocent people, and still he briefly believes that the single good thing that may come out of that, is that their people are safe, their friends - and they are both alive, too. Perhaps, they could do it together; try to mend each other - work towards what they don’t think they deserve, which is happiness.
Then, she says that she’s not going to stay, and that knocks the air out of his lungs. She kisses his cheek (an action that tells him nothing more than how sorry she is). She hugs him, and he knows that she’s in pain - for Christ’s sake, he is as well, but his face is hidden from her view. Yet he grabs on to her hair as if trying to keep her from slipping through his fingers, and he wants to talk but the words won’t come out - instead, they get stuck.
(I’M NOT CRYING! YOU’RE CRYING)