Depression can result from a faulty perspective. My depression was triggered by the break-up of my relationship with a girlfriend. That in itself should not have resulted in depression, but I discovered that, even though I had been a Christian for quite a few years, my life had become focused on my girlfriend rather than on God. I had let my spiritual life slip, and when she departed, it seemed I did not have much left to live for. Since that time, I have learned that it is only in putting God absolutely first that we can receive the grace to really love other people.
Depression feeds on itself.
My depression soon took on a life of its own. I became depressed about a whole series of issues unrelated to the initial trigger. I was depressed about the widespread suffering in the world, about the hollowness of political ideologies, about whether I would ever be able to earn a living over the course of a whole lifetime. Knowing what I know now, I assume that the initial depression caused a change in the chemistry of my brain that then led to further depression. I did not know any of that then.