Pinkie Pie wets her shorts for fun. She absolutely loves the warm, wet feeling spreading through her skintight shorts and flowing down her legs, as well as the idea of anyone wetting themselves being funny to her. After she's done peeing, she'll sit in her puddle for a bit before taking a bath in her wet outfit. She'll probably fart in the tub before she's done. Maybe she'll even laugh at the bubbles in the tub and pee again!

An improved version of this image, which wasn't a very good edit, especially her stream. I cringe every time I look at it so I decided I should do a better version of it.