I didn't quite get this part at the end of the latest episode.
Is it a custom to write a letter to Celestia once you earn your cutie mark or is it because they're closely related to the mane6? And why does Luna take such special interest in this case, almost as if the letter was for her?
>why does Luna take such special interest
"HOLY SHIT, THOSE LITTLE RETARDS ACTUALLY DID IT!"
I'm going to guess it's because she's had several dream interactions with them, and therefore there's some friendly, maybe guardian/sisterly/motherly figure connection there.
>why does Luna take such special interest in this case
if you really need to ask, you dont watch the show.
That was a Luna/Apple Bloom episode.
There's also Sleepless in Ponyville, which has a subplot about Scootaloo and Luna.
And of course For Whom the Sweetie Belle Toils which dealt with Sweetie Belle and Luna.