I am infuriated by the "all in your head" school of denial of mental illness, or the "just get over it" approach. My very simple response to these sorts of crashingly ignorant attitudes is to point to a line which occurs, as far as I'm aware, in basically every definition in the DSM, and which says "Causes signifcant impairment in school, work or social functioning." For there, you see, is the rub.
A mental illness is not just behaviour we don't like or aren't used to, it's behaviour we aren't used to and which seriously fucks up the sufferer. We lump them together in diagnoses based upon shared patterns of symptomatology not because we have the definitive proof that they actually are related (physiologically, we don't), but because this allows us to talk about patterns of behaviour that respond in a similar fashion to certain treatments, and therefore guides us in which treatments we should be using.
Over at Science-Based Medicine, Harriet has written the best post I think I've ever seen which summarises these matters and explains why psychiatry is important and needs our research dollars. If you want a more in-depth look, Steven Novella links to a five-part series he wrote on Neurologica about mental illness denial.
The problems discussed by Harriet are the usual problems which will always plague any school of medicine which can't show you before-and-after X-rays, and are ones which even I have made reference to over the years - flawed studies done using the inherently difficult-to-research subject matter of people; the inability to do a blood-test for mental illness (can't do one for migraine, either); the problems with the DSM, an inherently political and cumbersome document (which, incidentally, contains shifting diagnoses that have lead to a large part of the hysteria about the so-called and completely fallacious "autism epidemic") and so on.
Psychiatry remains like many other fields of medicine - the practitioner is important, the relationship between the practictioner and the patient is crucial, and just like your GP when they don't have access to blood tests and want to play on the safe side, medication is an inexact science.
But people who deny treatment because they deny the existence of psychiatric disorders, or are terrified about suidical ideation, are firmly in the same category as people refuse other forms of medical treatment - they are dying basic human rights, and perpetuating abuse through neglect.
Link to entry "Psychiatry Bashing" on Science-Based Medicine