Why, Pinkie Pie? Why?
Fluttershy is sad tree.
If you don't understand the joke, it comes from the pony meme Fluttershy is a tree.
The scene is from part of Season Two Episode 18: "A Friend In Deed". Here's a picture: [link]
I could have made this into a three-panel comic, but I didn't because
I'm a lazy bastard
I only needed one picture to say everything.