Rainbow Dash makes it through the ceremony but she can't handle seeing Applejack with Rarity, the woman if her dreams. So she escapes to the dressing room to have a breakdown. She gets support from an unexpected source.

Note about the prequel, it's mature, but that's only because it uses profanity over a small stretch if that doesn't bother you it's still a good read. But all you need to know from the prequel that RD is in love with Rarity.