Brooding 24/7 leads to disinterest, as it may inspire some to say "get over it," or "here we go again with the 'oh how dark my world is' routine." Part of the charm of The Avengers movie is that Captain America can be as upright/"light" as a character can be, but he plays well against sour Tony Stark, struggling Banner and noble/arrogant Thor. In other words, the contrasts offer a chance for each character to show what he's made of against someone who sees life in a very different way. If all of the Avergers were bitter, brooding and dark (i.e. a gang of Nolan Batmen), there would be no appeal, as its a one note character trait only a few can relate to.