Freud initially saw the phenomenon as an obstacle: patients reacted to him in ways unrelated to anything happening in the consulting room. The contribution that lasted is that he quickly recognised it as not an obstacle but the most directly available material.
The logic is simple. A relational pattern formed early activates in any sufficiently significant relationship, and the therapeutic relationship is built to be exactly that. Someone who felt constantly judged by a parent will hear judgement in a neutral silence. Someone who learned that closeness is followed by abandonment will turn distant precisely when things are going well.
What makes the therapeutic relationship different is that the pattern can be observed in real time, together, instead of merely recounted. That is what therapy offers and a conversation with a friend cannot: a setting in which the reaction can be examined without the relationship breaking.
Countertransference is the therapist's reaction to the patient. It is not a professional failing — it is an instrument, provided it is recognised. That is exactly why supervision is mandatory in most training schools.