Toska - noun /ˈtō-skə/ - Russian word roughly translated as sadness, melancholia, lugubriousness.
No single word in English renders all the shades of toska. At its deepest and most painful, it is a sensation of great spiritual anguish, often without any specific cause. At less morbid levels it is a dull ache of the soul, a longing with nothing to long for, a sick pining, a vague restlessness, mental throes, yearning. In particular cases it may be the desire for somebody of something specific, nostalgia, love-sickness. At the lowest level it grades into ennui, boredom.
Vladimir Nabokov, cited in A F
We ask everything of love. We ask it to be anarchic. We ask it to be the glue that holds the family together, that allows society to be orderly and allows all kinds of material processes to be transmitted from one generation to another. But I think that the connection between love and sex is very mysterious. Part of the modern ideology of love is to assume that love and sex always go together. They can, I suppose, but I think rather to the detriment of either one or the other. And probably the greatest problem for human beings is that they just don’t. And why do people want to be in love? That’s really interesting. Partly, they want to be in love the way you want to go on a roller coaster again — even knowing you’re going to have your heart broken.
-Kafka on the Shore