Everypony has a guilty pleasure or two. Those things about yourself you hide from even your closest friends. For some, it’s a simple book or song that they’re embarrassed about liking. For others it’s a secret hobby they don’t want others to know about. They make you feel bad, but you can’t stop enjoying them. Can’t deny you like it.
Rainbow Dash’s guilty pleasure is Twilight. Even if she hates herself for it. Even if she knows it’s wrong.
After all, what would Rarity say?