Bellamy doesn't think he's seen that sort of smile out of Clarke before -- definitely not on their Earth, with the way things had been going.

He looks away and back up to the sky at the mention of Finn. That, when Bellamy thinks about it, had been the real start of when Clarke started to slip away from them. They all grieved, but Clarke had been so caught up in it with almost no time to heal, and she'd steadily moved further and further away from Bellamy, toward Lexa, and finally beyond the Grounders entirely.

He's turning back to Clarke, about to say as much, when a flash of light in the sky catches his eye.