The thousand-year curse has caused them pain for life after life.
She is a devil; he is a god. She was the son of gods and demons; he was the son of the king of gods; they were not supposed to be together, and could not even meet. However, because of the heaven-defying love of the previous generation, they loved each other for all their lives, but could not be together for all their lives. She will eventually die in his hands.
He didn't know why, but she knew the cause and effect, and knew the curse, but she could never express it.
Even if this is a lifetime curse, even if I know we can't be together, I still won't regret it. Just because...
"I love you..."