Why does "Your grace" use "Your"?
Solution 1:
This is from the OED:
- Chiefly with capital initial. a. With a possessive adjective: a title of respect, esp. for a person of royal or noble rank. Frequently (in your Grace) as a form of address. Now archaic or hist. Formerly (in England until the reign of Henry VIII and in Scotland until 1707) used for a monarch or prince; now replaced by Majesty or Highness (see majesty n. 2, highness n. 2b).
b. spec. As the title given to a duke, duchess, or archbishop. (Now the only non-historical use.)
Here is a snippet from the etymology of it (which is very long) from there: Etymology: < Anglo-Norman grase, graze, Anglo-Norman and Old French, Middle French grace, Anglo-Norman and Middle French grasse (Middle French, French grâce ) favour, benevolence, an instance or manifestation of favour, (specifically) the benevolent influence and favour shown by God to mankind (all end of the 11th cent.), (chiefly in plural) thanks (1135), talent, virtue (second half of the 12th cent.), (in plural) prayer of thanksgiving after a meal (c1160), pardon, mercy, forgiveness [etc. etc.]
With the idea of the divine right of kings is also the idea of By the Grace of God.
The only person above a king was God. And the kings were also subject to God's Grace, in that sense they were like all others. So, I suppose that historically, there came a time when this identification of the king as having God's "special" grace was turned into a form of address.