In the Marvel Cinematic Universe, one of the most shocking moments came with Captain America in Avengers: Endgame. It occurred when Chris Evans' Steve Rogers decided to pass on the mantle in the finale. That scene revealed that after returning the Infinity Stones, Steve stayed in the past with Peggy, and then his older self arrived to gift the shield to Sam Wilson.
Many wondered why he'd remain in the shadows, though, as it felt uncharacteristic of Cap to know all the drama to come yet stay hidden. Still, others felt it was an emotional, rewarding choice and the mark of a man who deserved happiness and a shot at true love with his soulmate. Well, thanks to Thor: Love and Thunder's Mjolnir twist, Steve's swan song and his overall sentimentality for leaving the soldier's life behind might have been explained.
In Love and Thunder, the hammer reacted to Jane Foster, reforming after Hela shattered it. Mjolnir turned Jane into the Mighty Thor, allowing her to become a superhero in the Odinson's absence. Interestingly, Jane initially thought it could help stave off her Stage 4 cancer.
But in Love and Thunder's final act, it was revealed Mjolnir was actually doing the opposite. It was draining Jane's life-essence, having reacted to her based on a promise-turned-spell Thor cast on it to always protect her. It seems as if only Asgardian gods could really wield it, which is why Jane's mortal essence was dissipating the more she used the hammer. As such, Thor forbade her from her final transformation for the fight against Gorr at the Eternity temple, only for Jane to join the battle and eventually die.
Now, it's a pretty dark turn, but this could inform Steve Rogers' choice after he rocked a hammer Thor stole from the past in Endgame. Cap wielded Mjolnir to battle Thanos, proving worthy as he called down lightning against the Mad Titan. Granted, it didn't change Steve into a Thor, but he was definitely channeling the Odinforce by commanding this ethereal strength. Thus, it stands to reason that even if he wielded it for a short time as a pseudo-god, Mjolnir could have sapped something from Steve's mortal frame too.
Like it did to Jane in Love and Thunder, Mjolnir might have altered Steve's body, actually weakening the Super-Soldier Serum. With its effects negated, Steve might have felt it within his body. If he woke up after the fight and realized something was off, Steve could then have made his mind up to retire, knowing he was no longer that Sentinel of Liberty of old. Becoming someone sort of 'normal' would have reminded him how fleeting life is and that he needed to experience something other than war. As such, sensing the change within but knowing other heroes were still around, Steve might have decided to remain in the past for a more human experience. Ultimately, Mjolnir may have affected his genetic make-up and mortality, pushing him to recognize a sign of destiny and choose something he wouldn't regret: a family.
Thor: Love and Thunder is now streaming on Disney+.