One man is lucky in love, the other man is unlucky. Check out this excerpt from the text (Raoul is speaking) to see the stark contrast:
Oh, fear nothing! you are beloved,- you are beloved, Guiche; do you feel the value of these three words? They signify that you can raise your head, that you can sleep tranquilly, that you can thank God every minute of your life. You are beloved; that signifies that you may hear everything,- even the counsel of a friend who wishes to preserve your happiness. You are beloved, Guiche, you are beloved! You do not endure those atrocious nights, those nights without end, which, with arid eye and consumed heart, others pass through who are destined to die. You will live long if you act like the miser who, bit by bit, crumb by crumb, collects and heaps up diamonds and gold. (28.93)
In other words, seeing Monsieur de Guiche happy in love is almost like a torture for Raoul, who remembers how depressed he is because La Valliere rejected him. De Guiche being happy in love simply emphasizes Raoul being unhappy in love – this is a textbook foil.