Psychiatry isn't a social science of hating?

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Psychology happens to be a course that I was interested to take prior to learning minor bits prior to coming to the class. The unit we are currently learning about is Mental Disorders, and according to our teacher, she says that there is a huge book that Psychologists use called the DSM. I want to know, how does a Psychologist/Psychiatrist correctly diagnose someone having this disorder and someone not? If the Psychologist/Psychiatrist incorrectly diagnoses the person who in reality doesn't own this disorder, then will there be any reprimands made to this mistake? Not to be against society, but in a sense, these disorders label people. It marginalizes them from "normal" people, and at the end of the day, how does a psychologist/psychiatrist know whether he or she is normal or not? Just a thought..