Humanity is just a state of mind
"You'll die," Milhaust says, hopeless, helpless. "Princess--"
"I'll die either way," Agarte tells him, passionate in her hyperbole. "I will die of a broken heart, you know I will."
He has loved her, and loved her well, but always from afar: she the goddess of dawn, beautiful and immortal, he her faithful but lowly servant. Men laud him as brave and honourable, forever faithful to his duty. Milhaust wonders if it is truly honour and courage that bind him and not cowardice, fear of loving his princess in spite of the consequence. She is the daughter of the God of War, granddaughter to the Earth and the Sky; he is a mere human. It is arrogance to love her, and so he loves distantly.
And yet despite this, she offers her immortality to him, a man, whose life is so fleeting and meaningless in the span of her own. A man, who will wither and die. A man who is so far below her to be beneath any possible notice.
"Princess," he implores, "Agarte. You cannot throw your life away for me. What will your father say?"
"What should I care?" she says, scornful. "If you love me, Sir Milhaust, I would be anything; gladly would I give up my immortality, so that I may age and die alongside you. And if you do not, then I would rather die right now than live a thousand years more without you."
And he could never deny her anything, so he says yes: yes, consigning her to a life cut short, and he to the wrath of the heavens.
There is a time for cowardly honour and a time for couragous disgrace, however; and Milhaust knows that there was never any other choice he could have made.