This is actually pretty enlightening to Celestia's and Twilight's relationship. Original canon for the show listed our favorite adorkable nerd as the only one to actually receive the dragon hatching test. I guess this is the reason Spike is the only dragon running around in pony civilization.
And Celestia is supposed to be mysterious, I guess we sort of came to that conclusion though! Hopefully we get to see a bit more of her in the future.