There are many factors that play a role in mental health conditions, which cannot all be covered by diagnostic manuals, recommendations and guidelines. In the end, much depends on personal observations, clinical experience, information on similar cases, and selecting what seems important.
Symptoms are largely subjective with few reference points. The diagnosis often depends on the focus and the diagnostic system one uses. Since the symptoms reported by patients are subjective it is difficult to operationalise them. There are different diagnostic manuals, such as the DSM-V and the ICD-10, which, while they agree on many things, do differ. Adding to this millions of pieces of potentially relevant information, variations in focus and even the patient’s partial lack of access to information about himself or herself, makes the application of a straight forward scientific approach difficult.
Since one usually does not know the exact probabilities, one has to compare observed patterns heuristically to compare up with a new theory or an insight that may apply to the specific case. Impressions from memory and (relatively) free associations play a role in the process, which is reminiscent of the artistic process.