I disagree about Superman. He doesn't just defend the Earth because he can. He defends it because he already lost the world of his birth, lost his entire species, and he's determined to make sure nothing like that ever happens to his adopted home and people. It's not really that different from Batman -- he has to live with the loss he couldn't prevent, and he deals with that by doing all he can to keep it from happening again. True, he had a happier upbringing because of the Kents, but that inconceivably vast tragedy in his past still has a powerful impact on him, and his status as the only living Kryptonian brings a certain loneliness with it, motivating a powerful drive not to lose the people he cares about. The key difference is that it's more personal for Superman. Batman is driven to make sure other children don't lose their parents, but the people he's compelled to protect are mostly strangers -- and if anything, the people he is close to are the ones he draws into his fight and shares the danger with. Superman has more of a family, both adoptive and extended, and is closer to the people of Metropolis and the world, so he sees it more as taking care of his family and community. But they share a similar compulsion to prevent loss because of their personal experience with loss.