Because the song goes "'Cause I want to make you grin, grin, grin... (Yes, I do)".
I froze-frame that scene from "A Friend in Deed" because I thought that was one of Pinkie Pie's best expressions (in one of her best songs, of course). So here you go. A nice big vector*. She wasn't in the air during the scene, but she wasn't shown in-full either, so I improvised.
*Bigger vector can be uploaded if you guys want it. But let's not go nuts.
"My Little Pony: Friendship is Magic" & Original Characters © Hasbro.