(NOTE: This is not a ship-fic. No romance here, just somepony trying to cheer up her friend. Move along. Nothing to see here...)
Based on the scene at the very end of "The Last Roundup": Pinkie Pie and Rarity are left stranded in the Badlands, their long and agonizing journey home requiring them to spend a night alone in the desert. It is here that the latter discovers a painful secret hidden beneath her friend's constant cheerful demeanor.
Despite not being the one famous for bringing a smile to everypony's face wherever she goes, the exhausted (and most unacceptably filthy) fashionista does everything in her power to make her happy again...
(Now with a sequel: link)
(Submission for Equestrian Harmonics' second group contest, prompt #1)
(Featured on EQD and added to Twilight's Library)
(A million thanks to Pre-reader 63.546 from EQD for helping me turn this story into something decent!)