In some pieces, you keep learning more about it for many years. Psychoanalysis is a medical technique, a form of therapy for the mentally ill or depressed patients that helps them understand the root of their problems. For many reasons, though, these techniques are of interest to literary critics. For one thing, analysis of a patient using these techniques is a matter of interpretation. This interpretation is often of a rather literary kind.
Analysts also rely on symbols and images the way a critic interprets a work of literature.
Freud himself often turned to literature to illustrate his famous theories. Freud's use of the Oedipus Complex was based on the play Oedipus Rex by Sophocles to illustrate Freud's theory that young infants, both boys and girls, have a natural erotic attachment to the mother. He stated that as the infant ages, he gradually comes to realize that the mother is not sexually available because she is already erotically attached to his father. In Sophocles' play, Oedipus unwittingly kills the father and marries his mother. According to Freud, the father becomes a rival to his son as he grows, to the point that the child thinks of killing his father so that he can possess the mother.