[bolding mine]

I think that last one is the key there: because pre!serum Steve had spent his entire life observing the world around him — he had to, right, because when you grow up sick and asthmatic while the US is in the throes of the Better Babies/Fitter Families eugenicist movement, you’re Othered: because you’re a narrative of failed masculinity, because you’re a potential “pollutant,” because you’re a failure to your country (I don’t think enough people really appreciate how hard Bucky had Steve’s number when he questioned whether enlisting was about proving something, or just how significant it was that Erskine chose “the little guy”).  Steve tells Peggy, “if you start running, they’ll never let you stop” — that’s a realization born of experience and painful, constant, nervous observation: of someone who’s not just constantly picking fights but likely also having them hoisted on him.  Pre!serum Steve grew up in a situation where he would have had to learn to read the body language and situations around him as a coping mechanism.  I don’t think it’s an accident that pre!Serum Steve was an artist.  He was an observer: he paid attention.  He likely knew his world better than most.

In 2014 he’s in a world that makes no sense to him.  It’s not just that it’s new: it’s that he’s suddenly not invisible — not just because he’s Captain America, but because he’s broad, handsome, big and coded as a masculine ideal; it’s because he’s afforded all of those privileges in how the world sees him that he never had before.  It’s that his grief has kept his world of observation narrow (it’s why he can lie to Pierce and Rumlow — both situations where they’re superiors or equals, where he’s likely observed them over time; learned to read them) but has no idea what to do when he’s approached by a friendly stranger, why he finds the idea of Natasha’s cover — that they’d be getting married, that Steve would be marrying a woman like her — makes him giggle like he’s terrified.  It’s why he panics when Natasha tells him to kiss her, because he has plenty of bloody, first-hand experience with how the world he left behind reacts to a 5’5,’ 97-pound version of Steve Rogers, and maybe people haven’t even changed that much, but he’s never had any idea how they react to this version of him (remember how well he did confronted with female interest, or the moment with Bucky in the bar, in TFA).  

I think this is also a part of why he falls apart so completely when he sees Bucky.  This is someone who’s supposed to see Steve, to know him, in any situation — who’s supposed to wade in when Steve can’t.  And Bucky looks at him and (to Steve) sees nothing.  And now nobody knows who he is.