Captain America can lift Thor's hammer because he is deemed "worthy" by the enchantment on Mjolnir, which states that only those who are worthy can wield its power. This worthiness is about moral purity, honor, and righteousness rather than raw strength. Captain America, aka Steve Rogers, embodies these qualities and has been shown in both comics and the Marvel Cinematic Universe (MCU) to be one of the few who can lift the hammer. In the MCU, this was hinted at in Avengers: Age of Ultron when Captain America was able to slightly budge the hammer, and fully realized in Avengers: Endgame when he wields Mjolnir in battle. The Russos, directors of the Avengers films, and Marvel Studios president Kevin Feige have explained that Captain America was always worthy but chose not to lift it earlier out of respect for Thor. Another theory is that Steve Rogers became fully worthy by the time of Endgame due to his acceptance of his own motivations and the truth he confessed in Civil War, which restored his moral purity. Thus, Captain America lifts Thor's hammer because he is one of the few truly worthy characters, possessing the honor, integrity, and selflessness required to wield Mjolnir's power.
