The thing is, Steve and Bucky’s friendship wasn’t just a “they grew up so they’re as close as brothers,” thing, it’s way more dynamic than that. Their relationship may be symbiotic, but it was never particularly balanced. Growing up, Steve was sickly and physically weak but had precisely the same personality as he does now: unendingly hopeful and moral, with no conception of backing down from a fight. Hence why he’s constantly getting beaten up, with Bucky having to wade in and finish those fights for him. Bucky is incredibly protective of him, which must have caused a fair amount of confusion when Steve turned into a near-indestructable supersoldier.
It’s a classic partnership between idealism (Steve) and pragmatic cynicism (Bucky), with Bucky playing the role of Steve’s protector and #1 fan. It takes the supersoldier serum for everyone else to realise what Bucky knew all along: that Steve is an inspirational figure, destined for great things. There’s this beautiful transitional moment in the first Cap movie when Steve has just rescued Bucky and the other soldiers from the Red Skull’s lab, and Bucky calls out for everyone to cheer for Captain America. This is basically the first time Steve has ever received outside recognition for being the person he is, and you can tell that while Bucky’s happy for him, he’s also a tiny bit resentful because he knows that Steve is no longer “his”. The tables have turned, and now Bucky is walking in Steve’s shadow, rather than the other way round.
Before, Steve had resented Bucky for his physical strength and the way he was able to get into the army while Steve was repeatedly marked down as physically unfit for duty. After he becomes Captain America, the tables begin to turn, with Bucky probably recognising that Steve’s strength and heroism comes at a price (to their friendship, at least). But in the end that resentment between them doesn’t matter, because whichever way you flip it, they define each other. CATWS purposefully sets up tons of parallels between them — Steve falling from the Helicarrier just like Bucky fell from the train, Bucky being defrosted, Bucky being strapped down in the chair like Steve was when they gave him the serum, Bucky being physically “improved” in a twisted mirror-image of Steve’s supersoldier perfection, the pair of them facing off along that walkway on the Helicarrier — which serve as a constant reminder of how closely their stories are intertwined. Even if Bucky isn’t onscreen, he’s still always lurking in the background, out of the corner of Steve’s eye. [x]