system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] How do elves change species to men like Arwen did?

Can any elf do this? Why aren't men allowed to turn into elves?
gpt
Only half-elves are granted this choice, i.e. the decendants of Beren (man) and Lúthien (elf) and Tuor (man) and Idril (elf). Pure elves and pure men aren't given this choice. 

Elrond chose to be an elf, and his brother Elros chose to be mortal. Elros descendants weren't given this choice though. It seems that once half-elves have made the choice to go mortal they can can never go back.