He made an everlasting promise to her, "We will be together forever!"
For him, she would rather turn into a lonely ghost, carrying the danger of being wiped out in ashes, but she has no regrets, and is only willing to continue the love relationship in the seventh life and keep the unchanging promise throughout her life.
Is it because they are too obsessed, or is it a trick of fate, or is this a mistake in the first place...