Final answer:
Freud was interested in the unconscious mind when patients showed symptoms without physical causes, which led him to develop psychoanalytic theory and methods like dream analysis and free association.
Step-by-step explanation:
Sigmund Freud became interested in unconscious personality dynamics when he noticed that certain patients' symptoms did not seem to have a direct, physical cause. Freud developed his psychoanalytic theory based on the idea that the unconscious mind plays a critical role in influencing conscious behavior. Hysterias and neuroses, along with various other symptoms, were thought by Freud to be connected to repressed memories and unresolved conflicts deep within the unconscious. His methodology included dream analysis, slips of the tongue, and free association as a means to access the unconscious mind. Freud's work greatly emphasized the importance of early childhood experiences and has had a profound impact on the field of psychology despite criticism and modifications of his original theories.