Mental illness that is constitutional, what used to be called psychosis, may be patholigized, but it is hardly as common as some shrinks seem to think.
There is also another source of mental illness, probably the most common one. In an age that receives the words "sin", "vice", and "immoral" with amusement, discomfort, indifference, or even outright hostility, we seem to have lost the ability to confront this cause of mental illness, namely, sin, vice, and immorality.
There is no crisp line between physical, mental, and moral maladies, so to speak. And in this respect, vice can produce what used to be called neurosis. Sins and vices harm the sinner, and when left unacknowledged and undealt with, the guilt can manifest in a variety of bizarre ways that twist our emotions, appetites, motives, perceptions, and reasoning. It can reinforce cycles of vice. Pathologizing can be one way to find temporary, illusory relief by scapegoating one's guilt by invoking "chemical imabalance". Sure, there may be a chemical change in the brain, but that change in this scenario is the effect and not the cause.