When you love someone who loves and marries another

 "As if I would nurse my resentment, Nastenka! As if I would drive a dark cloud across your bright, serene happiness, as if I would inflict misery on your heart with my bitter reproaches, wound it with covert pangs and make it beat anxiously in your moment of bliss, or crush even one of those tender blossoms woven into your dark curls, when you go with him to the altar . . . Oh, never, never! May your sky be clear, may your sweet smile be bright and happiness, which you gave to another, a lonely, grateful heart!
God in heaven! A whole moment of bliss! Is that not sufficient even for a man's entire life? . . ." - Fyodor Dosteovsky, White Nights, Page 56

When the Object of our Affections do not Love us in Return

 "Heavens, how joy and happiness lend beauty to a person! How the heart overflows with love! You seem to want to pour all it holds into the heart of another, so that everything turns to gaiety and laughter. And how infectious that gladness is! Yesterday the words held so much comfort, such kindliness towards me in heart . . . how she danced attendance on me, so affectionate, how she cheered and soothed my heart! Ah, how flirtatious sheer happiness can be! And I took it all at face value; I thought she . . .
Good lord, though, how on earth could I have thought such a thing? How could I have been so blind, when it had all been appropriated by another, none of it was mine; when eventually even that same gentleness of hers, her solicitude, her love . . . yes, love for me - was nothing but gladness at the imminent prospect of a tryst with another, an urge to thrust her happiness on to me too? . . . When he failed to arrive, after we had waited in vain, she fell to frowning, she quailed and lost heart." - Fyodor Dosteovsky, White Nights, Page 38 and 39

This is a short story by Dosteovsky about a man who falls in love with a woman who seemingly lets on only herself to be in love with another man.