Fluttershy's been feeling overstretched lately, and she really needs a break. She wants to go drinking, but she can't go alone. Luckily, she has a long standing deal with Rainbow Dash: any time she wants to go out and have fun, Rainbow Dash will go with her and make sure she's safe. How this strange system came to be is another story, but Fluttershy is eternally grateful to her friend for following through every time she's feeling stressed. In fact, the only thing wrong with this system is that Fluttershy can never remember what happens the next day.
But Rainbow Dash knows that's a good thing.