How we cite our quotes: (Part.Letter.Paragraph)
Quote #4
The girl is more confiding, or, which comes to the same thing, more talkative, than her discreet admirer. (2.53.2)
One quality that distinguishes Valmont and Merteuil from the other characters is the care they take with their language. Every word has a purpose, usually to manipulate the reader or listener. While most others reveal themselves by what they say and write, these two work hard to conceal their natures with their correspondence. They're very good at it. Teenaged Cécile, though, doesn't have much of a filter.
Quote #5
Vanity and friendship have been dictating my letter, and they are both chatterboxes. (2.63.19)
Don't be fooled: when the Marquise is long-winded, it's because she wants to be. This is a really clear statement of how the Marquise can tell how her letters are probably being interpreted.
Quote #6
I was, at all events, as unreasonable as I was capable of being: for there is no showing tenderness without talking nonsense. It is for this reason, it seems to me, that women are better writers of love-letters than men. (2.70.8)
The Vicomte is a proud misogynist, so it shouldn't surprise us that his scorn for women incudes the way he believes women typically use language: nonsensically. Women are irrational and emotional. No surprise, then, that women get the better of him. His "logic" didn't foresee it. This is a theme of about a zillion love stories: the rational, unfeeling guy gets unexpectedly swept off his feet. Think about Mr. Darcy, who struggles in vain not to fall in love against his better judgment.