Who gave Sigmund Freud authority to be an expert on dreams?
If there had been someone who could have given Sigmund Freud the authority to be an expert on dreams, who would have given that other person the authority to give Freud the authority? Freud would have been glad to meet someone who had such wisdom and authority--but such a person did not exist in his time. He studied most of what had been written about dreams in several different languages, going all the way back to the Pharoah's dream about the seven fat cows and the seven lean ones which was interpreted by Joseph in the Old Testament. Freud spent many years exhaustively analyzing his own dreams. He is credited not only with being the first person to understand the language of dreams but the first to realize the existence of the unconscious.
Freud was a physician. Many of his patients were women who suffered from symptoms that had no apparent physical causes. He came to understand that physical symptoms, such as paralysis, could be caused by the unconscious. This led him to focusing his great intellect on trying to understand the unconscious, and he came to realize that dreams were, as he stated it, "the royal road to the unconscious." So he was mainly interested in understanding dreams as a means of understanding the unconscious processes that resulted in all kind of serious physical ailments.
Dreams had been a subject of great interest throughout human history, but Freud considered himself the first person who really understood what they were and how they could be interpreted. He believed that the function of dreams is to preserve sleep by representing conscious and unconscious wishes as being fulfilled. Ub other words, all dreams are wish fulfillments and frequently the fulfillment of repressed wishes. He was his own authority on dreams. That was what made him one of the greatest thinkers of his times, along with Einstein and Darwin.