Dan Didio asked Mark Waid why Superman did what he did and he said in part:
"He was raised as one of us, but he's really not one of us. Superman is the sole survivor of his race. He is an alien being, and he is probably more alone in this world than anyone else ever has been....
Kal-El knows instinctively that it is only when he puts his gifts to use that he truly feels alive and engaged....
Only by being openly Kryptonian can he also be an Earthman with exuberance and excellence. When he lives as who he really is, in full authenticity to his nature and gifts, and then brings his distinctive strengths into the service of others, he takes his rightful place in the larger community, in which he now genuinely belongs and can feel fulfilled....
Yes, Superman aids those in peril because he senses a higher moral obligation and yes, he does it because his natural instincts and Midwestern upbringing drive him towards acts of morality--but along with that genuine altruism is a healthy amount of self-awareness and a surprisingly enviable ability on his part to balance his own internal needs with the needs of others in a way that most beneifts everyone."
I think, btw, that this still applies if he doesn't know his exact origin. The "he's not one of us" is obvious even without that, as would be the loneliness. He's the only person with his gifts.