In one way, it's hard to be angry all the time. It takes energy, burns calories, requires passion. In others, it's the easiest thing in the world because there are many valid reasons to be angry: injustice, prejudice, persecution (personal and impersonal), poverty, disease... And there are degrees of anger: annoyance, righteous indignation, rage, and the most extreme kind - Hulk.
When Rogers says, "Doctor Banner, now might be a good time for you to get angry," he decides it's time to trust his friends with the Other Guy. "That's my secret, Captain: I'm always angry."