definitions of what is "normal" change with every new edition of the manual, as a result of new research findings and greater awareness, as well as lobbying efforts and political factors. A strong cultural bias exist.
it isn't always clear which diagnosis fits you best, so your therapist may have to do some guesswork or fill in some information gaps just to fit you into a diagnostic category.
Insurance companies also dictate what conditions they do and do not cover.
if you're seeking couples therapy, chances are your therapist will have to diagnose you or your partner with a mental disorder such as depression or anxiety in order to fulfill insurance requirements.
Lobbying efforts from pharmaceutical companies can dictate the prevailing diagnoses.