(Trigger for strong emotions.)
I guess everyone would like to know the answer to that question. And nobody really seems to know. It is attributed often to the "match" between therapist and client -- but what does
that mean? What are the factors that you can identify that promote a "match"?
OK, I have THE answer!!!
It is really very simple. Not mysterious, not something incomprehensible to the mind of man (or other human). A therapy can succeed if the therapist is truly and completely
mindful of his or her own feelings. If a therapist can recognize without hindrance, without internal censorship, what those feelings or emotions are. Only then can a person distinguish between
things that are and
feelings about those things. That is all there is to it.
The problems arise when feelings cannot be recognized. When that happens, one begins to confuse "feelings" and "things". One begins to use words that mix the two, and mix them
while not realizing that they are being mixed.
Here is an example, which probably will bother people because they think it should be a valid use of the word: beautiful. I contend that word actually is a mixture, and an
unrecognized mixture, of "things" and "feelings about things". No "thing" is "beautiful". Things do not have feeling-qualities. They just "are".
People (and other creatures) have feelings
about things. The feeling that something "is" beautiful really means that you have a very nice feeling about it. Seeing that thing produces a sense of "congruence" within you. Congruences feel good. That's the way the brain and system works. Dissonances feel bad. Those arise when the system is unable to process what the
feelings about things are. And why does the "system" fail to process successfully (mindfully) those feelings? Because we have been penalized, often "to say the least" for having those feelings. They have been suppressed in the service of avoiding fear, fear of punishment, fear of abandonment by a person of significance (read parent).
If a therapist cannot freely access her or his own feelings, and to the extent which that failure occurs, no therapy is likely fully to succeed. The "match" between therapist and client reflects how damaged is the ability to recognize what one's actual feelings are
in both the client and the therapist. The more damaged the client's system the less impaired the therapist's system must be to produce a successful outcome.
OK. That's it.

This is a subject that arouses big feelings within me -- because I have been penalized by people whom I took to be "authorities" for actually realizing what my feelings were, and expressing them where that was not wanted. Those "authorities" included most significantly my mother and my first therapist. Neither wanted my actual feelings. My mother "penalized" by beatings, among other things. My therapist "penalized" by attributing to
me thoughts and feelings which were
his and not mine. I accepted his characterization of me into myself because he was the "authority". It resulted in my massive breakdown (for which he blamed me).
OK, I guess that was not "it". Mistakes have consequences.