Why did you give up that shield? Why are you making such a big deal out of something that has nothing to do with you?
Sam’s expression says it all. He feels for Bucky. And he feels for Steve; he respects the decision he made in handing that shield over to him but Sam, as he points out in his next line, also knows that the shield carries more than Steve anticipated when slung on the shoulders of a black man. Sam, at this point, has already checked Bucky’s privilege once before (re: this is what you’re not going to do. you’re not going to […] tell me about my rights). He can see that Bucky is in pain, in limbo, in need of confirmation that he is on the right path, or that he’s even capable of getting onto the right path. Steve gave that to him. Steve knew him, and believed in him. In the same way he believed in Sam. And rightly so. Everything Steve saw in Sam is very much there. And in someway… Sam doesn’t doubt that. He doesn’t doubt that Steve wanted him to be the next Captain America. And he probably would have wanted to carry on the legacy if he didn’t already know exactly how this shit would turn out. And honestly? Bucky should have an idea by now, too. Sam has already shut him down once. Bucky has already seen first hand the fate of black heroes (see: Isaiah Bradley). He literally just witnessed racial profiling with his own two eyes and has some small insight into what life must be like for Sam on a day to day basis. And he still doesn’t accept that the decision Sam made to not take the shield had nothing to do with Steve, nothing to do with Bucky. And everything to do with the way his entire life has been like up until now, with no hopes of it really changing in the near future — at least not enough that Sam can even think about being the one to instigate that change, rather, would likely lose all ability to make a difference and help people if he did actually take up the shield. So Sam’s expression? (And god, Mackie’s acting is just PHENOMENAL, Seb’s too)! There’s sadness there. For Bucky. Because his friend has been through a lot and is struggling with so much still. But there’s sadness there because his friend is so completely oblivious to Sam’s own struggle, and has unknowingly made it about himself.