An "armchair psychiatrist" is a person whom is not trained in, or has no background knowledge of psychiatry, but will still continue to give advice concerning, and often attempt to diagnose, the mental health of others.
I believe that armchair psychology is a problem because the armchair psychiatrist is often under the impression that diagnosis of a person's mental state can be simplified down to a list of symptoms. It seems to me, armchair psychiatrists often see mental health diagnoses as capable of being easily sorted into neat categories by using a set of simplistic lists of symptoms.
But, I feel, this particular attitude towards mental illnesses, in many ways, trivialises mental health conditions as those whom are often the givers of this sort of "advice" seem to fail to understand the true complicated nature of mental illnesses.
I think people feel more confident to give uninformed diagnosis' of mental health conditions, as opposed to physical illnesses, due to the non-visibility of mental illnesses, which to a layman can make mental illnesses seem possible of diagnosis by observation simply by use of lists of symptoms.
I believe another large contributing factor towards the simplistic views of armchair psychology is the regularity of so-called "experts" giving their opinions on the mental health of those in the public eye, or "celebrities", despite the fact that the discussing "expert" would never actually have had any contact with the person under discussion; certainly not in a context where they would be able to provide an informed opinion of that person's mental health!
Because, in reality, if this person really was under the care of the "expert" giving their "informed" diagnosis - or, to term it more accurately, their opinion - of another's mental health, then said expert would be bound to the rules of confidentiality and, therefore, would be unable to provide any public comment of the condition of that person's mental health.
Armchair psychology can be a problem due to the simplistic nature of an armchair diagnosis, particularly as the arrogance (for lack of a better word), often displayed by an armchair psychiatrist in the accuracy of their highly uninformed opinion of complex mental health conditions, not only trivialises mental illnesses; but, also, to a layman, a seemingly simple and easily reached diagnosis can make treatment of a condition also seem simple and easily accessible when, of course, this is often not the case.
I understand that engaging in armchair diagnoses of complicated mental health conditions can be a fun past-time or activity, but armchair psychology can have a damaging effect especially when an armchair psychiatrist begins to condemn another for disregarding their advice concerning their mental health - regardless of the fact that the armchair psychiatrist is, ultimately, wrong in giving their uniformed and/or simplistic "diagnosis" or opinion, of a complex condition.
Armchair psychology simplifies mental illness - and, in many ways, trivialises it - as it instills confidence (or arrogance) in what is generally an opinion born out of ignorance, or out of a lack of understanding and knowledge, of either mental health conditions and/or the person under discussion proceeding an armchair diagnosis.
This, I feel, is the problem with armchair psychology.