Interesting point. The thing I've always kind of liked about Superman is that he seems, to me, to do the things he does not out of any personal tragedy or motivation, but simply because it's the right thing to do - he was raised with strong moral values, and he has these incredible powers, and he feels that this is the only right way to use them. The whole 'Krypton exploding' thing is where he comes from and gets his powers, but it's not his origin as a superhero - he doesn't really have one. He just does what he does because it's the right thing to do, as a conscious decision. But, like I said, that's my perspective. The concept of 'I was saved, therefore I must save others' is a distinct possibility - however, I would point out that, in both modern and Pre-Crisis continuity, he learned of his Kryptonian origins AFTER he became Superman, not before - so I'm not sure if it holds up to scrutiny. (Post Crisis, he learned that he was an alien before he became Supes, but not from which planet he was from, or the circumstances of his journey.)