True Love.
(Author's Note: I am an unabashed lover of positive, well-meaning, and KIND Superman. That is what the character is intended as, a man with powers above and beyond all of our understanding deciding to be human and like one of us. Not to rule us as a god, even though he could, but to support us and love us.
Henry Cavill, through absolutely no fault of his own, could not be that Superman. The failing of Man of Steel, and to a lesser extent BvS, is due to Zack Snyder. You cannot be a Randian disciple and believe in the concept of a Superman. There is simply no way. So, with this story, I give that Superman a talking to from the exemplar of those ideals currently: the Arrowverse's Superman, played expertly by Tyler Hoechlin. This takes place after Crisis on Infinite Earths, where the DCEU-verse heroes save the day.)
It had been a violent, paradigm-shifting battle. Throughout the multiverse, heroes had assembled to follow their most basic mission, the one that supersedes all others: protect the world. Except, this time, the world meant not just their own, but every other world that existed in this strange thing called the multiverse. But as Clark Kent from Earth-38 looked around, noticing his exhausted compatriots from all different earths, he admitted to feeling a kind of joy at seeing just how many of the people he had fought alongside were wearing his house's crest. He supposed it meant something, something bigger than he could even fathom, that the lessons of the House of El remained strong even when all hope was lost. But as he looked around, he saw something that made him frown.
He saw someone wearing his crest, who had his powers, and yet was seeming to hold himself apart from everyone else. As the heroes from other Earths started to congregate, Clark couldn't help himself. He pulled this other Superman aside, and looked into his eyes with earnest hope.
"On your earth, you are their protector are you not?" Clark of Earth-38 said, gazing at this man who he was going to call Kal-El just to keep his mind about the whole thing.
"They scorn me, and mock me. I do not protect them" said Kal-El, resentment dripping off of every consonant and vowel.
"Do you love them? Would you give your life to see them saved, to have them know how much you love them all? That is your failure, Kal-El. Take it from someone who knows, who feels the power and love of humanity on a daily basis. If you cannot see the best of them, you will dishonor what our fathers wanted for us. You will not be the man he wanted, the man Jonathan Kent and Jor-El raised us to become."
And with that, Superman from Earth-38 walks through his breach and back to his home world. He just hoped his lesson, the things he had to learn, was going to bear fruit.