...there is nothing more difficult in love than expressing in writing what one does not feel...
[Il n'y a rien de si difficile en amour que d'écrire ce qu'on ne sent pas.]
In Letter Thirty-three, the Marquise de Merteuil comments on Valmont's seduction-strategy against the Présidente de Tourvel. She criticizes him for choosing to write to the Présidente instead of employing some other method. Valmont's writing, she says, will eventually reveal his true emotions.
This quote is especially important because it encapsulates one of the most important themes of Dangerous Liaisons, that of sincerity. As we know, the Marquise and the Vicomte are never intentionally sincere. Their concern is always whether they have the upper hand. There is no such thing as sincerity in their version of love. One might even claim that there is such thing as love where they are concerned, except if love is a combat zone, instead of an emotion. Love is an opportunity for competition, not togetherness. Therefore, if one writes of love, one must appear sincere only to lure the other person in, but one must remain insincere, so as not to be taken in oneself. This contradiction produces the difficulty of which the Marquise wants to warn Valmont.