People who are disappointed by Steve’s reaction or (apparent) lack thereof after Bucky dissolved really don’t grasp Steve Rogers at all. This isn’t like fanfic. In fanfic, Steve sometimes, sort-of, semi works through his problems in a sometimes dubious but mostly healthy way.
Canon Steve doesn’t do that.
This is a man who buried his last living relative and was ready to refuse to live with Bucky because he didn’t want to appear weak.
This is a man who, after watching someone he loved literally slip through his hands and die, separated himself from the undoubtedly well-meaning Commandos, and Peggy (until she ultimately sought him out) and cried and attempted to get drunk in a bombed out bar because he was their leader and he couldn’t, wouldn’t let himself fall apart in front of the men that follow him into battle and trust him with their lives.
This is a man who has grown used to the weight of the world on his shoulders, and he now feels that weight more than ever. This situation with Thanos is so much more than any situation has ever been, in every way.
So he cannot be Steve Rogers. He cannot mourn for his best friend, he cannot scream at the top of his lungs, or pull his hair, or weep, or wail, or break. Despite how much he might want to. Despite how he feels. Because, whether the official title is his or not, whether he wants it or not, he needs to be a captain. He needs to be The Captain. With Stark gone, with his people – everyone he trusts and loves – gone… if you think for a second Steve Rogers is not going to beat back his anguish and pain immediately after suffering tremendous, soul-rending loss to do what needs to be done, you are dead wrong.
An alternative to being disappointed, is to really think about who Steve is. He is self-sacrificial to a fault; he swallows the knives of duty with a bitter smile until he all but chokes on them, keeps going even when blood runs down his chin and his throat is in shreds. Emotional, yes, he feels deeply, but only when it’s “safe” to; when he doesn’t have to be on, when he doesn’t have to be anything other than body, and mass, and fractured soul. Unfortunately he doesn’t feel safe that often. A soldier, yes, that’s who he was trained to be, but inherently, that’s just who Steve is.
His soft “oh g-d” at the end – about Bucky, about Sam and T'Challa and everyone – was as much emotion and devastation and fear as Steve Rogers was going to let himself show. Just a second, just a moment. And then… Then he’s going to get up, he’s going to throw his shoulders back, and he’s going to go back to work.
