A mental health diagnosis is often received as the discovery of an underlying condition. It is more accurately a decision that a particular pattern of symptoms fits an agreed description.
Diagnosis in psychiatry is descriptive rather than mechanistic
In much of medicine a diagnosis eventually points to a physical finding: an organism, a lesion, a measurable abnormality.
Most psychiatric categories have no such marker, so they are defined by the presence of a specified number of symptoms over a specified period.
The category is therefore a summary of what is observed, and two people with the same label can present quite differently while both meeting the definition.
The groupings exist to make communication and treatment possible
Without shared categories, clinicians could not compare cases, services could not be planned, and treatments could not be tested against a defined population.
A diagnosis also functions administratively, determining eligibility for services, funding and, in many systems, insurance cover.
These purposes are legitimate and they shape the categories, which is one reason diagnostic manuals contain boundaries that look arbitrary from a purely clinical view.
Overlap is expected rather than anomalous
Because the categories are built from symptoms, and symptoms are shared across conditions, a great many people meet criteria for more than one at the same time.
Disturbed sleep, difficulty concentrating and low mood appear in several definitions, so their presence narrows the possibilities much less than it appears to.
The frequency of multiple diagnoses is not a sign of unusually complicated patients. It follows directly from how the system is constructed.
Categories change between editions
Diagnostic manuals are revised periodically, and revisions add categories, merge them, adjust thresholds and occasionally remove them entirely.
Each revision is made by committees weighing research evidence, clinical utility and the consequences of change for people already diagnosed.
A person's diagnosis can therefore change without anything about them changing, which is disconcerting but follows from the nature of the definitions.
What the label does and does not tell you
A diagnosis usefully indicates which treatments have been studied in people with a similar presentation, and it gives access to services organised around it.
It does not explain cause, predict course in an individual, or describe the person, and treating it as though it does is a common source of distress.
Anyone uncertain about a diagnosis or how it was reached is entitled to ask the clinician who made it, and formal routes for a second opinion exist in most systems.