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.
There are a number of other features of our modern world that seem to have contributed to this, not least in that we live increasingly isolated lives. The generations have been separated and families spread out as our society becomes increasingly mobile.