Sometimes the muse awakens from her slumber and things happen.
Oliver and Diggle have barely reached the bottom of the stairs before Felicity is racing toward them. John barely has time to step out of the way so she can fling her arms around Oliver's neck. He's seen this kind of thing a thousand times before: two people wrapped in each other's arms, saying nothing. Not just from them but from his military days.
Every homecoming there were women desperately clutching at their returned soldiers, as if seeing them wasn't enough to proved that they were really there. And those soldiers, perhaps not gripping as tightly but their closed eyes and contented smiles just as meaningful. John always liked seeing that. It made him feel at peace. Like everything would be OK. But it only makes him worry whenever he sees Oliver and Felicity in the same situation.
Because this is as open as they ever are with each other. In a moment, they'll let go. They'll go back to the quiet détente that's been going on for months because they're both too afraid of someone getting hurt that they won't take a chance. He worries that they're hurting more this way. He worries what will happen to her if one day Oliver doesn't come back. He worries what will happen to him if one day Felicity isn't waiting for him at the bottom of the stairs.