Allow me, if you will, to put some science into the science fiction of the marvel cinematic universe.

the shield super soldier serum functions as an imperfect non sentient symbiote

it’s a unique blend of cells that requires a host to survive, when it enters into the system of the host, it immediately starts taking over and in a series of rapid cell multiplication and deletion, starts to make things better by replacing the old cells with the serum enhanced cells

however the cell replacement is imperfect in that it replaces healthy host cells fine, replaces its own cells very well, but damaged tissue is difficult for the serum to recognize and replace to optimum functionality

Steve had weak muscles but they were healthy muscles and therefore they were able to be enhanced by the serum, it took longer for his lungs to operate at full capacity but Steve had medication to assist the function of his normal lungs and so between the medication and the serum they were able to get those lungs up and running pretty quickly

Now what about damaged/imperfect cells that don’t heal on their own, even with the super soldier serum? This treatise posits that they remain the same in that the super soldier serum cannot determine how to fix these cells and so it settles with imitating the cells to the best of their enhanced abilities

Operating under this assumption, it can be inferred that the scars that Steve had pre-serum stayed with him after the serum was introduced into his system

Insert a transition sentence here!

Prior to the sleek vaccine system we know and love involving a singular needle depositing the vaccine intramuscularly, the vaccines, specifically the smallpox vaccine in this case, that were in use around the time when Steve was wandering around being all pre-serum and adorable were administered through scratches into the superficial layers of the skin created by multipronged lancets

As such, anyone who received the small pox vaccination were left with a distinctive scar on their arm

So obviously Steve was like what’s this something to make me less sick? Sign me the fuck up and of course he drags Bucky along and then a half hour later they’re walking out of the clinic with perfectly circular wounds on their left biceps

Of course they laugh: chicks dig scars! need me to kiss it better? I'll give you a lollipop if you don't cry this time. and these little scars that they share feel more like badges of honor, brotherhood, and love than they feel like representations of their inoculations

Now when Steve rescues Bucky from hydra imprisonment Bucky is obviously thrilled to see him but also he’s never seen Steve like this before, he finally has the physical presence that is attitude always needed and now it’s like he’s transformed. Bucky always knew Steve was brave but seeing him in battle is this cognitive dissonance that he can’t really get over for a while.

That is until one night they’re sleeping rough with the commandos camped out in the middle of nowhere and they get shoved into the same tent because dammit Steve snores and Bucky is the only one who can sleep through it and so they’re lying together, Bucky trying to find a way back to his friend that suddenly he doesn’t feel like he knows any more

Bucky is running his fingers over the new Steve just trying to familiarize himself with this new body when he feels that little nickel sized depression in Steve’s skin and by god it’s night out and he can’t see a damn thing but Bucky would know that scar anywhere in the world because he has the exact same one. And so their matching scars become almost a talisman to remind them of who they were and who they are to each other.