What I think a lot of people are missing is that the entire notion of mental illness is a hack. The basic definition of any mental illness requires that it interferes with ones ability to live a normal life. Resist the urge to deconstruct that for a second, I'll come back to it.

Some people have problems that some type of professional help can enable them to solve. This can range from people who need medication to avoid seeing and hearing things that aren't there to people who need CBT to get out of a self-destructive rut to people who need cosmetic surgery to feel like their body fits their subjective gender. But the United States is full of systems where you need a diagnosis to do anything--bill insurance, prescribe medication, declare someone to have a disability so they can collect income without working. So DSM is a tool for psychologists and psychiatrists to put a code down on a form so the system will let them help people.

This is why there's a stipulation that it's only a mental illness if it prevents you from living a normal life. Lots of people have the symptoms of mental illness, just not to such a degree that it warrants intervention. Having compulsive rituals is perfectly normal--compulsively washing your hands until they bleed is not. A good mental health professional will know the difference and only provide a diagnosis when it's necessary. But ultimately the DSM isn't a bedrock of abnormal psychology, it's just a tool psychologists use to make the system let them help people.