Shining Armor was actually relieved to find that you wanted to volunteer. Apparently, it had been a whole different kind of hell trying to search for somepony who actually wanted the job; no one wanted to do it! Celestia had several candidates in mind, but Shining was just happy that you volunteered, and since you were a human, the background checks were fairly swift. Changelings could only disguise themselves as four-legged beings, which made sense since the dragons and griffons also walked with four legs. Humans weren't on the list, especially since you were the only known human in the world.