I think that if you look at heroism as a binary state, and you find a hero, then you're either going to find that they're heroic by nature or that one day they just spontaneously became heroic for no satisfactory reason. The only way to get someone who evolves into heroism by dint of personal merit (whether we're talking about effort or virtue) is to either look at the process from the inside, presumably when writing a PC background; ditch the binary and look at a whole lot of gradations; or recast it as perceptual, which is to say, not when someone IS a hero but when you/your PC/various mechanics recognizes them as one.
The Exalted rules mostly deal in it as a binary, or at most a scale with six points. I don't think that's bad, and the standard drama-as-physics assumption means that those breakpoints are meaningful, but it's probably misleading if you're going to think critically about what a hero really is.
Put another way, if you want the human spirit to spontaneously overcome the circumstances grinding humans down, you're either going to be able to explain how it happened or you're not; and if you can explain it, either it's going to be something mechanistic, something metagame, or an arguable metaphysical assertion about how the real world works.