My favorite take on why immortality would suck was just a bored dude floating through space lol. Earth's not gonna be here forever.It might not be jackpot. Being immortal isn't all it's cracked up to be. Check out Takahashi Rumiko's Mermaid Saga.
This is exactly what it Implies.I thought the twist was gonna be that some person or corpse got caught in the net and an impressionable child sees human parts mixed in with fish parts and her first thought is "I'm eating a mermaid."