Dangerous Liaisons by Pierre Choderlos de Laclos

Indeed you are very kind not to abandon me to my melancholy fate: the life I lead here is really fatiguing, from excess of repose and insipid uniformity. Reading your letter with the particulars of your delightful excursion, I was tempted twenty times to pretend business, fly to your feet, and beg of you to commit, in my favour, an infidelity to your Chevalier, who really does not deserve his bliss. Do you know you have roused my jealousy? Why tell me of an eternal rupture? I recant an oath taken in a fit of frenzy. We should not have been entitled to so solemn a privilege, had we seriously intended to keep it. Ah, may I be one day revenged in your embraces, for the vexation the Chevalier's happiness gives me! I am all indignation I own, to think that a man who has scarce common sense, without taking the least trouble, and only simply following the instinct of his heart, should find a happiness I can't attain. Oh, I will disturb him: promise me I shall disturb him! But have you not humiliated yourself? You take the trouble to deceive him, and he is happier than you. You think you have him in your toils, but you are in his. He sleeps quietly, whilst you wake for his pleasures. What could his slaves do more?

Hark ye, my lovely friend, while you divide yourself among many, I am not in the least jealous; I then look down on your lovers as on Alexander's successors; incapable of preserving among them that empire where I reigned sole monarch; but that you should give yourself up entirely to one of them, that another should exist as happy as me, I will not suffer; don't expect I'll bear it! Either take me again, or take another; and do not, by any exclusive caprice, betray the inviolable friendship we have sworn to each other.