David Hume (1711-1776) was, amongst other things, a philosopher. In his essay Of Tragedy (included in the Four Dissertations volume) he explores the reasons why humans are so keen on tragedy.
Tragic plays include scenes of joy only to better plunge the audience into the next dramatic act. But push the audience – or reader – too far, and they become immune to the effects which should be just enough to ‘excite’ the heart.
So the advice to authors: draw your readers on, weave tales of sadness, melancholy, tragedy and disaster, but keep a tight hold on the reins. For although absence makes the heart grow fonder, extended absence deadens the heart to the joys of the return. So you should keep the reader dangling on tenter hooks, and reward them with an emotional story that won’t release them until the final chapter.