Depression Can Literally Kill the Brain

I remember a conversation I had with a friend about a year ago. We were debating the merits of a theory I had. This was that if someone were to convince themselves of a reality different than that of the actual world, if it caused a sort-of happiness, the ends were justified by the means. Now, while I don't believe that today, for some glaring fallacies of logic found therein, the following quotation does underscore the importance of harnessing control of your thinking habits.

Of all the discoveries that recent research into depression has brought to light, this may be the most disturbing. Depression [...] affects the hardwiring of neurons. To what extent the damage can be undone is still unknown. In the process, the brain loses its adaptability [...]. Our ability to feel fades, and our judgment and concentration diminish. Tests have shown that depressed people are less able to solve even simple tasts, such as the sorting of playing cards. At the beginning of a depressive phase, the working memory is affected, and the stress hormones impede the brain's ability to think. [...] The brain sends stress hormones, which can cause harm to the neurons and do lasting damage to the brain. [...] In people who have suffered repeatedly from serious depressions, the space in the prefrontal cortex occupied by certain kinds of neurons is diminished by a third [...]. Other parts of the brain lose so much matter that they just shrivel up. This has been observed, for example, in the hippocampus, on which memory depends.

-Stefan Klein, The Science of Happiness, pg. 188