Not every little quirk is a mental illness. You need a whole set of related quirks to be properly diagnosed. The DSM is by no means perfect, but its the best we've got at this time. There are several psychologists who want to convert to a spectrum process of diagnosis, rather than the checklist system we have now.
I don't really going around to every person and saying, "You just washed your hands. You have OCD!", "Did you hear what I said? Do you have ADHD?", "You cried last night? You must have depression.", "Your child did not speak to me. He must be autistic.", "You're still upset about that argument? You have a Histrionic Personality Disorder."
To do this would be highly unethical. That is why the DSM criteria exists. Diagnosis is not usually made until a clinical interview and/or testing is performed.
Also, keep in mind that many people seek psychological help because they are having problems with their "quirks". They require insurance to pay for meds and psychotherapy, but the insurance company won't pay a dime unless they have a diagnosis.
BTW- psychology is NOT easy and not exactly the profession you seek if you want to make a fortune. And consult with any neuropsychologist and they will tell you that many mental illnesses are due to biological factors (i.e. frontal lobe formation in ADHD).