First, psychiatry has a real problem with pathology. In order to determine that something is in fact a disorder, you first need a normative model. But what exactly is psychiatry's normative model, and is it a good one? Not only that, but history has shown that the field is highly vulnerable to political pressure (the evolution of the DSM is a great example). Let us not forget that the norms and demands of the consumerist culture are unnatural and those who choose to live by that culture will measure themselves in accordance with it. There is an objective reality, but culture can either enhance our view of it, or corrupt our understanding. Psychiatry is a part of culture and thus not only not immune to culture, but a product of it.
Second, the range of symptoms can be both improbably expansive and mutually contradictory to such a comical degree that such purported disorders begin to look like a blank canvas onto which we can project practically any real or imagined problem. When you are experiencing problems of unknown etiology, and someone comes along and gives it a name, it is very tempting to embrace the diagnosis because of the relief it seems to bring, that this condition has a name, and not only a name! Salvation is at hand! But that relief does not demonstrate correctness.
Third, the lines between the psychological, the physical, and the moral are blurred and fluid, so looking for smoking guns in one domain is bound to lead to frustration. Some factors may be constitutional, such as temperament. We know that certain temperaments lend themselves better toward certain kinds of work and styles of work than others (to draw on classical terminology, you would much rather have a phlegmatic judge rather than a choleric one; a sanguine waiter rather than a melancholic one). Some factors may be developmental (the psychodynamics of the environment we grow up in leave a mark on us all, for good and for ill; confused or wrong ideas about what's true and what is good). Some factors may be moral (bad habits, prurient interests, a lack of temperance, vices of all stripes, the resulting repressed guilt). All can contribute to our inability to focus or function. However, I claim that, especially with the excessive medicalization of all human discomforts and problems, real or imagined, the moral is the most neglected and arguably the most important. It is the moral, the proper exercise of practical reason, where our power to choose, rightly or wrongly, ultimately rests and the bad practice of which leads to enormous problems in also the psychological and the physical domains.