Because that's what this mirror does.

[He knows it's not a good answer, but Chuuya's mounting apprehension is somehow unsettlingly calming. It's as though someone here is supposed to be that agitated, that defensive and skeptical, and it opens the door for Fawkes to relax because it means things are progressing exactly as they're meant to.]

It grants a wish — no, not just a wish. A heart's wish, its fondest desire. All it asks is that you trade your life for it.