Those Three Big Words
Blair and Chuck start off continuing their entirely unrealistic friends-with-benefits arrangement, when they find themselves making a dinner date (I can't be the only one suddenly obsessed with lobster pot pie). But when they're each urged to stay away from each other for their respective images, they break their plans. They're both OK with the decision until Chuck slips out the three words you're not supposed to say to the girl you're having no-strings sex with.
Though Blair tells herself they were words uttered in the heat of the moment, a romance novel and delivery of lobster pot pies is all she needs to convince her that she feels something too. So she obviously rips her dress off to go find Chuck.