The UK borrowed the term ennui from France during the height of 18th century European Romanticism and I'm so glad we never returned it. It was used to describe a rather fashionable kind of weariness, boredom and dissatisfaction with the world and a preoccupation with the emptiness of existence.
From the moment we wake up until the moment we go to bed, there are so many things we can get annoyed with: traffic noise
Sometimes there is another, legitimate, explanation for a person losing ten pounds, working late and losing interest in sex. I just don't know it.