This site has a nice summary definition of transference and countertransference. Drive theory, ego psychology, and object relations' theories focus on the intrapsychic, and the unit of attention is the individual. Interpersonal theory focuses on two-person psychology and the relational field as the defining force for personality and psychopathology. Problems in people's lives are due to their interpersonal realities. Sullivan's own troubled past greatly influenced his beliefs and outlook.
Relationships provide a sense of personal value.
Loneliness represents the most severe human emotion. Like Freud, his theory evolved from his clinical practice. In Sullivan's view countertransference reflects all the emotional reactions of the therapist in relation to the client. Sullivan saw anxiety as interpersonal in nature. He was very interested in the concept of self-esteem.