Evans’ Steve Rogers has become the heart of the MCU, and that moment—when it still seems impossible that this little guy could ever be putting on a superhero costume to save the world—proves it. With those six words, the movie establishes that Steve Rogers would be putting his life on the line to help people and to stand up for what he believes in even if he never became super strong and fast. Plus, after he does become strong and fast, he still holds on to this “never back down” attitude, dropping the exact same line in big fights with the Red Skull (later on in The First Avenger) and with Iron Man (in Captain America: Civil War). Later on in the movie, when Stanley Tucci’s Dr. Erskine is ready to choose Steve for the experiment that will turn him into Captain America, he tells Rogers that the most important thing is not him becoming the perfect soldier. It’s him staying true to who he is, “a good man.”

The other MCU heroes who had already been introduced have their own motivations for why they want to help people, whether it’s Tony Stark’s guilt over selling weapons for his whole life or Thor’s genuine love of adventure and smashing villains with his hammer. But Captain America just wants to be a good man because it’s the right thing to do. “I can do this all day” is simply who he is, super-soldier serum or not.