Avoid deep dark wells and/or abysses of pain. In characters, I mean. Well, in real life too, but that kind of goes without saying.
That advice is from a blog post by Heather McF.
One of the reasons for writing fiction is to explore the highs and lows of life. Quoting myself: “writing can be a pleasurable exploration of a wonderful landscape“.
Our human brains allow us to experience the heights of elation and the depths of despair. From the perspective of evolutionary psychology we can ask if even our lowest moods have a function. It is possible that mild depression is adaptive and has been selected evolutionarily as a brain mechanism that can shift our behavior out of maladaptive ruts. When we experience anxiety, helplessness or frustration our brains perform a cost-benefit analysis of the situation. Our brains are designed to become obsessed with important problems and either solve them or show us that we are wasting our time on a problem we cannot solve.
Our brain mechanisms for dealing with problems that we cannot solve do sometimes go “off the rails”. It is a tragedy for an individual if they cannot move out of a “pit of depression”. It is a tragedy for readers if an author mercilessly leads us into a “pit” and never shows us a way out. In contrast, a successful author will have descended into the depths, done battle with demons and returned to the surface hand-in-hand with their Muse, ready to describe some glorious revelation that allowed escape from the “pit”.