My theory about depression.

We usually think of depression as a negative force but I like to think of it as a positive influence, and my* theory is that it has its roots in an evolutionary survival instinct developed in all creatures.

It goes like this: When an animal is under attack by a predator, it instinctively loses interest in everything else - the food it was grazing, its surroundings... the only thing it can now think of is escape and survival.

As we evolved into humans and became more intellectual in our pursuits, this instinct too evolved to the intellectual level. Hence when we are "under threat", for example insecurity borne of personal or material loss or lack of opportunities, we get into a frame of mind where, among other things, we lose interest in everything around us.

Our whole mental focus is now on whatever is bothering us and on ways to fix it. We should use this mental state to single-mindedly find solutions to the problem at hand.  Unfortunately, it often goes the other way, we just keep thinking of the problem and not the solution, and that pulls us down into the depths of despair and possibly destructive tendencies.

* I have not heard of this anywhere else, but it is possible I haven't looked hard enough.