Kiyo heard of the mermaid's ability to give immortality, and became immortal himself as he's wedded by a merman. They both share the curse of immortality. With this newfound way of life, Kiyo became unsettled and sought after the way to die, and as a merman's spouse, it isn't an easy treat. After learning about the only way to die from the Racoon Youkai, and the tragic meaning for his merman having wedded a man, Kiyo dropped his will to die and picked up a resolve to live until eternity with his merman.
(The ending made me yell out because it's so nice.)
Wadatsumi no Yometori