Once Steve gets over the shock of seeing Bucky again, once he’s thinking about what Zola did, all he can hear echoing through his mind is
I should have known.
He should have known because when he pulled Bucky out of Zola’s lab, he could barely walk, but he marched back to base at Steve’s side, under his own power.
He should have known because none of the injuries Bucky suffered in the field took the way they ought to have.
He should have known because while the Howling Commandos fought their way through Europe, Bucky was always at his side, always keeping pace with him.
He should have known because Bucky shouldn’t have been able to keep up with him, and he always, always did.
He should have known, but he didn’t – because his whole life, he’d been trying to keep up with Bucky.
And it never once occurred to him that Bucky shouldn’t be able to keep up with a super soldier. Because Steve was the super soldier, and of course Bucky could keep up with Steve. Because Bucky had always been ahead of Steve, and Steve’s new body just meant he could catch up.
It just meant Steve and Bucky were finally side by side, just like they were always supposed to have been.