You're using incorrect language there. It creates a bar where a person has to show "this many symptoms" in order to be diagnosed with a particular illness. This is how modern medicine works and none of us question this model when it comes to physical illness so why should it be different for mental illness ? For example flu-like symptoms alone should not be enough to diagnose someone with the ebola virus or AIDS and the same principle applies to mental illness.
>It creates a fear of being told that your problems aren't really problems, and you're just not dealing with them correctly (regardless of the intent of your original argument).
No. It creates an expectation that one should refrain from automatically assuming that normal human experiences are indicative of mental illness. It's normal for human beings to experience certain problems (such as fatigue, anxiety, trouble sleeping, weight gain/loss, etc) during the course of their life and it doesn't mean they are necessarily sick.
The view that "almost everyone is sick but most people are hiding it out of fear" is a ridiculous notion that is probably being injected into the cultural conversation for commercial reasons and I doubt many people take it seriously.
It's never been more socially acceptable to start taking drugs for mental illness due to the pervasiveness of drug advertising. I think it's easy to make an increasingly strong argument that it's been over-normalized to the point where misdiagnosis is commonplace
Go read the DSM-V, or the DSM-IV. (In fact, don't, but at least read up on how they work.) They are the diagnosis guides which are used as a baseline across the USA, and in fact for some things across the world.
They quite explicitly list out symptoms, and how many of them one has to have in order to be diagnosed as having a mental illness, and even within those a scale which allows doctors to prioritise care.
I think that one can end up promoting self-diagnosis and over-diagnosis simply by using the term "mildly depressed" to refer to the general idea of "being upset" or "being in a bad mood" which is becoming increasingly common in our vernacular. I would argue that it's becoming increasing common because commercial interests set out to promote it's use in that way.
The problem is that your argument, even if it is true for a significant portion of people diagnosed with mental illness (which I personally am not convinced of, but also have no hard numbers on hand), makes it more difficult for those people who currently are afraid of how society will react if they attempt to treat their illness. It creates a bar that someone has to be "this sick to get diagnosed". It creates a fear of being told that your problems aren't really problems, and you're just not dealing with them correctly (regardless of the intent of your original argument).
I think that it's not appropriate _right now_ to deal with over-diagnosis as a serious issue in comparison to under-treatment. Maybe, in a future when it's socially acceptable to get help for mental illness, we can deal with over-diagnosis if it's a serious issue. But trying to stamp out over-diagnosis right now contributes to a more dangerous stigma.