It depends on which storyline you're talking about, so consider this a generalization: Superman feels loyalty and gratitude towards the humans that raised him as their own after his planet was destroyed (along with his parents) and wants to repay their endless love and kindness. He does this by trying to improve the general welfare of the people of Earth, which makes his Earth-parents proud of him. But he doesn't have to help us. He helps the people because he is honorable and believes that the strong should stand up for and protect the weak. Or something like that. I never was the biggest Superman fan.

superman doesn't exist

Gary Gary Gary!!