I believe that Superman leaves because of the appearance of General Zod and his cohorts on Earth. After he defeated them he thought that there may have been other survivors and travels to the remains of Krypton to find out for himself. He comes back and tells man Kent that "it's just a graveyard."That would explain why he leaves and gives more depth to his character. He is alone in the universe. After his bust with Lois, he realizes that he can't be just a man, but doesn't want to be alone as Superman. He goes in search of others like himself to no avail. He returns to Earth to pick up what pieces of his old life are left.