Rarity has found that she's fallen in love with one of her best friends. Well, there a few things that can be done in a situation like this: Try and push the feelings aside for the sake of your friendship, or you just tell them how you feel and hope for the best. In Rarity's case, she decides to go with the latter. After considering the ways AJ could react, or the way it could affect their friendship, she decides she wants to let her feelings be known regardless if they're returned or not.
Though, she'd prefer that they were returned.