A psychological diagnosis is about meeting behavioral and emotional criteria, functioning like a check list.  You look at the list of symptoms - persistent sadness, trouble sleeping, constant worrying, etc – you see how many of these symptoms the teen in question displays and if they have a certain number of them you assign the diagnosis.  It is not a difficult or complicated process.  It is almost exclusively descriptive and it does not contribute to my understanding of my teenage patients or how to help them.  It is not useful clinically.  Researchers use diagnosis as a way to run studies on large groups of people who, on the surface, have similar difficulties.  Insurance companies demand the assignment of a diagnosis in order to reimburse.