Often when I'm depressed, I find myself doing things which I know will make things worse: and the very reason I do them is because they will make it worse. The depression becomes a kind of living being, ensuring its own survival by advising and tempting me into various destructive and self-destructive actions (like starting fights with B for no reason, or shirking my responsibilities). Although I know what I should do in order to feel better, it does not appeal to me. I would much rather dig myself deeper into the pit of despair, and doing the opposite takes an immense amount of willpower.
There is something very pleasurable in destruction, even if what you are destroying is yourself. It must be an innate human tendency, because it's one of the first things young children do, as soon as they are mobile in fact. And once you have begun the destructive process, it seems not just easier but fated to continue (the whole "Might as well be hanged for a sheep as a lamb" argument).
Here's Dostoevsky's Underground Man on the subject (if you haven't read Notes from Underground, you should: it's both short and amazing):
I got to the point of feeling a sort of secret abnormal, despicable enjoyment in returning home to my corner on some disgusting Petersburg night, acutely conscious that that day I had committed a loathsome action again, that what was done could never be undone, and secretly, inwardly gnawing, gnawing at myself for it, tearing and consuming myself till at last the bitterness turned into a sort of shameful accursed sweetness, and at last—into positive real enjoyment! Yes, into enjoyment, into enjoyment! I insist upon that. I have spoken of this because I keep wanting to know for a fact whether other people feel such enjoyment? I will explain; the enjoyment was just from the too intense consciousness of one’s own degradation; it was from feeling oneself that one had reached the last barrier, that it was horrible, but that it could not be otherwise; that there was no escape for you; that you never could become a different man; that even if time and faith were still left you to change into something different you would most likely not wish to change; or if you did wish to, even then you would do nothing; because perhaps in reality there was nothing for you to change into.I think this sort of thought process is behind most mental disorders (addiction, eating disorders, anxiety, etc). That's why punitive approaches never work (boot camp for teens, "shaking you out of it", jail time, shaming, aversion techniques): they are just playing into the already acute sense of failure and degradation (and indeed might be just what sufferers are trying to obtain).
The only way out is love and forgiveness, of oneself and of others.