Love (in one of its forms):
Oh, my friend, her love for me was boundless and I loved her too beyond measure, but we were not happy. [A]lthough … we were decidedly unhappy together we could not cease to love one another; indeed, the unhappier we were, the more closely we were bound together. However strange this may seem, it was so.
~Fyodor Dostoevsky, from Dostoevsky: A Self-Portrait by Jessie Coulson