Not to grave-dig your comment anything, but what episode was that? And timestamp, if you know? I was curious what you meant and could not find it easily enough with just the information you provided above. Then again, I only checked the transcript of one episode, Just for Sidekicks.
The Crystal Empire - Part 2, towards the ending Rarity is sad because she wanted to remain as a crystal pony, it is then when Applejack says BUT THINGS ARE BETTER WHEN THEY ARE A RARITY to make her feel better, which really worked.
Sometimes, well most of times, one sentence is more than enough for the shippers to go crazy. XD