Pinkie we know she will act, remember of shining and cadance will have a baby but she most hold the secret, she mostly blow up and tell it, thanks of shining and cadance showing up and the plan for twilight finally get it. For Applejack, her she terrible on lie, she will soon tell it, Rariry she hold her self and will tell or make dress for celestia for a date, Rainbow her is gamble to hold the secret, she will tell or not tell. Fluttershy her she will shut her mouth of it but soon or later she will tell, this why is better is twilight know it!….but i have a bad feeling of this mystrieuse mare on the train, and she not rarity, her eyes not the one rarity have!

“Oh great, keep my friends blissfully ignorant and just throw the albatross on me, why don’t you Your Majesty?”

“They might tell the world my secret. And it’ll embarrass me.”

Well, someone could say that she is a little paranoic, but whe had seen what AJ and Pinkie did when they had a secret.
 
 
@Lurks-no-More
 
Poor Tia :c

The real reason, of course, is that Rarity would go nuts and pester Celestia to no end with dress and suit designs for dates, secret trysts, and finally wedding. :)