[HerenIstarion]

JRRT states that elven and human hroar, ie bodies, are genetically the same, hence possibility of interbreeding betwenn these races. that's fea, the spirit/soul, which makes the difference. When elven fate is changed to human, the body stays as it was, fea changes. Growing uneasiness of human soul, seeking the way to leave the circles of the world, makes it to leave the body = die.