How we cite our quotes: (Part.Letter.Paragraph)
Quote #1
But then what use is softening her heart if you are not there to take advantage of it? (1.33.3)
The Marquise de Merteuil is skeptical of Valmont's decision to seduce Madame de Tourvel more with letters than in-person. She thinks he should appeal to her heart instead of her head, and that his choice to write letters will put him at a disadvantage. Keep in mind that the Marquise doesn't actually want him to succeed. One reason it's so easy to be infatuated with someone you only know through letters or online is that you can imagine what they're like and fall in love with that, even if it has nothing to do with reality. So maybe Valmont's on to something.
Quote #2
There's nothing more difficult in love than expressing what one does not feel – I mean expressing it with conviction. It is not a question of using the right words: one does not arrange them in the right way. (1.33.4)
When Valmont succeeds with his seduction, the Marquise accuses him of really being in love and not, as he claims, faking it. She doesn't believe you can fake being in love; she thinks it's too easy to give yourself away by saying the wrong thing. Hey, it worked for these guys.
Quote #3
If you wish to keep up with this correspondence you must train yourself to decipher my drafts, because nothing in the world will reconcile me to the tedium of copying them out a second time. (1.34.12)
These were the days before computers with their copy/paste features and (gasp!) typewriters. If you wanted to make a copy of a letter, you would have to write it out again with pen, paper, and ink. Valmont takes the care to send polished letters, but he won't be making extra copies just so his frenemy the Marquise can read his handwriting.
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.
Quote #7
With Danceny himself I have often, in spite of myself, felt a certain constraint which prevented my telling him all of my thoughts. (2.75.4)
Cécile and Danceny have horrible communication skills—with each other, at any rate. They don't intend to deceive one another, usually, but they're not fully honest, either. They're communication isn't the calculated performance we see in Valmont and Merteuil's letters. It's more of an explosion of their fears and hopes and dreams. Cécile holds back because she doesn't trust herself not to spill her guts.
Quote #8
This perpetual harping on the same string, which I already find comes something short of the irresistibly amusing, must be exceedingly insipid to anyone who is not personally concerned. (2.76.20)
This line almost functions as an "aside" to the audience. None of us reading this novel are personally involved with the characters, and yet the author, Choderlos de Laclos, surely hopes his readers don't find the correspondence to be exceedingly insipid (bland, drab, boring).
Quote #9
This is what comes from using language which nowadays is so abused that it means even less than the jargon on compliment. It has become no more than a set of formulas, and one believes in it no more than one believes in 'your very humble servant'. (3.121.2)
The Marquise wants Danceny to write what he means—what he thinks and feels—not with flowery prose filled with clichés. Of course, she wants to know what he thinks and feels so she can manipulate him more easily. However, she can't give any indication of that, so she couches it as a stylistic point about the effective use of words.
Quote #10
But a letter is a portrait of the heart, and, unlike a picture, it has not that fixity which is so alien to love; it reflects all our emotions: it is in turn lively, joyful, at rest… (4.150.6)
This is Danceny writing to the Marquise, begging her to write to him. He's saying that that letters are very self-disclosing. He obviously doesn't know who he's dealing with, poor kid.