Mourning
If you only let oneself (yourself) into your heart, you'll end up mourning all of the loves and friendships you could have had but didn't let in.
Picture yourself all alone. You never let anyone into your heart but yourself, and now you're mourning the life and friends you could have had.
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While you're in mourning, you feel a sharp pain in your crying eyes. A key (き) comes sliding out with your tears. You look around wildly, but you're still alone. Is this... the key of mourning?
You try to push the key back into your body. Back into your eye, you even try to swallow it, but it comes back out. Finally you look down at your mourning heart and try the key there. It clicks. And opens.