marvel cinematic universe - Why isn't Captain America worthy of Mjolnir?

I haven't see the movie yet, but in the trailer, we can see Cap trying to lift Mjolnir in the humorous scene where every Avenger gives it a try.

Mjolnir even moves just a tiny bit and at this very moment we see Thor's smile disappearing. This, of course, is intended as a humorous and lighthearted scene.

But it got me thinking: why isn't Captain America worthy of Mjolnir? He seems to be the most "perfect" human being anyone can be: brave, not selfish, legitimate leader, prompt to sacrifice (as seen in the first movie, when he jumps on a fake grenade).

I ask this question in light of this very specific scene, but I'm interested in the Marvel Universe (and Marvel Cinematic Universe) as a whole. Maybe Captain America did wield Mjolnir once? Maybe he has some individual flaw I am not aware of that makes him unworthy?

Answer

The Russo brothers recently revealed that they interpreted the scene in that he could always lift it but just chose not to as we see in this answer by @galacticninja.

GeneralBenKenobi: Could Cap always wield Mjolnir (meaning he could have picked it up in Age of Ultron and decided not to), or did he progressively get more worthy over time?

Russo_Brothers: Anthony: He always could. Our interpretation of the famous scene in Ultron was that when he realized he could pick up Mjolnir he quickly chose not to, because he didn't want to embarrass Thor.

_{reddit, r/marvelstudios, We’re Joe and Anthony Russo, directors of Marvel Studios’ Avengers: Endgame. AMA!}

However, it is worth pointing out that before they said this Kevin Feige answered this in a reddit AMA stating that he could always lift it and was just being polite to not ruin the lighthearted moment that was that scene.

KrazzyDJ: Cap lifting Mjolnir was one of the strongest (crowd-cheering) moments in Endgame. Does he become worthy in that moment or has he been worthy for a while since, say, Avengers: Age of Ultron?

KevFeige: We think he was always worthy and was being polite in Age of Ultron.

_{reddit, r/marvelstudios, Hi reddit, I'm Kevin Feige. AMAA}

If we go into an in universe perspective we find out that Thor believed Cap could always wield Mjolnir. First off his reaction to seeing Mjolnir move in the scene in Avengers: Age of Ultron seems to show that he is aware of what happened. Later in Avengers: Endgame he says he was aware that Cap could wield Mjolnir back in that scene when we actually see it happen.

Thor: I knew it!

_{Avengers: Endgame}

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Just to address the other part of the question Cap has wielded Mjolnir on several occasions in the comics. The very first time he does this I believe is in Thor Issue 390 when Captain America wields Mjolnir in order to give it back to Thor. Later on after the battle Thor and Cap both hold Mjolnir's handle whilst talking about the bond that now binds them. In particular the last panel appears to be what you are looking for.

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Near the end of the Fear Itself comic series in Issue 7 we see Steve once again lift Mjolnir.

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After this point things get a bit weird and Cap wields Mjolnir in the Secret Empire series. To cut a long story short Cap is brainwashed by Hydra who have changed Mjonir's inscription to read:

Whosoever holds this hammer, if he be strongest, shall posses the power of Hydra.

It goes through some hoops but eventually Cap is freed and fights the Hydra Cap and in the battle wields Mjolnir before giving it to Lady Thor.

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The last instance in the comics that I am aware of is in What If? X-Men Age of Apocalypse where Cap wields Mjolnir all the way through. What is interesting is that in this story Cap can also use Thor's lightning powers.

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