Unhappiness in general is an evolved mechanism to tell us when a change is necessary. For more primitive societies this meant getting up and moving on from where you are, and expanding somewhere new. This is probably how humans were able to conquer the globe so completely in such a remarkably short period of time.
For modern humans, we cannot just pick up and make changes arbitrarily. There is no more space left, and social obligations keep us tied down. We can simulate some of this by changing jobs, getting more exercise, getting a divorce, etc.; but fundamentally yes unhappiness is a modern disease -- one there is no obvious cure for.
