He has never been truly happy.
He's tried of course, he's pretended. Even as a child something was calling to him, telling him he was different; he would never belong.
He's an alien, so far from home. His world is destroyed, his people are gone, and he is so very, very alone. He struggles to reconcile his past, his culture, with the ordinary world he's grown up in. He's young and ill-equipped, and feels positively inadequate for his position. He's shy and uncertain, and when around the girl he adores he's positively clumsy. He wants to explain, he wants her to help him. He needs her to fill the emptiness he feels, to take away the horrible aloneness, but he can't.
Krypton is gone, shattered into a million million pieces. Parts of it keep drifting down to earth, reminding him of what he's lost. A bit of metal, a few criminals, shadows of his father's name; the father he never knew. He tries to escape it all, to leave it behind. He tries to abandon what the crystal showed him. The world was fine before he came, it will continue without him as it did before.
He is so weak, so helpless. Nothing feels right. Nothing fits. This is not what he had hoped it would be. Even as he realizes he cannot escape who he is he knows that he cannot abandon what he has become. The people count on him, they look to him for protection. He cannot leave them alone now, he cannot abandon them. He has failed, he has betrayed them after giving them a false sense of protection.
He returns to his godship, to the cold emptiness of the stars. He swears he will not abandon them again. He will not betray them. They thank him, they worship him, but he shakes it off. He wishes they would not. He is nothing but lonely, afraid, and empty. He turns away from them, still smiling, and flies off into the night. Only then does he hide his face and weep, giving up everything he loves and desires for duty that has been pressed upon him. He will protect the earth.