Answer:
Take one royal word in the plural and make it singular by adding one letter
I'm the source of all emotion, but I'm caged in a white prison.
I have joy in bringing two together, but darning my existence! My life hangs by a thread, filled with ups, downs and resistance!
Why is 6 afraid of 7?
Sends paper without using the post office.
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.