It's no wonder that depression rates have skyrocketed. Mankind has come largely to deny one whole half of his own being: the soul. Simultaneously in deciding to dismiss this realty, they from then on ignore all of it's needs. Unfortunately, doing so has very real consequences.
I believe that there IS such a thing as mental illness that is actually not related to immorality, but I believe it is a lot, LOT rarer than that which IS linked to immorality.
It is part of our nature to seek happiness and want to avoid pain, so when a person desires strongly to live in sins, they must ultimately twist their minds like a pretzel excusing it, or making up fake reasons why they then feel depressed (a result of their sins). Some prefer an almost clean-cut departure from reality in general, to facing that they way they think, or the way they live their lives, is pure poison to the mind, heart and soul.
When the person is determined to go on sinning in the face of ever growing problems and misery caused directly or indirectly by doing so, the only thing left is either to quit sinning to stay sane, or else quit sanity to keep sinning. Unhappily, most people staunchly prefer the latter to the former. These heavily influential or mind-altering drugs are simply the next logical step in the futile fight to stay sane while one continues to sin their brains out, or ignore truths vital to our real happiness.
If you know a lot of young non-Catholics, chances are you've personally seen the ever rising problem of mental illness firsthand. But what struck me during the years I actually mingled with non-Catholics, was how very nearly ALL of their depression and mental problems could be traced quite obviously in many cases to sins and selfish, sinful living, or else to moral ignorance. These people need God, truth and a strong shot of reality, not drugs.