Why do so many people find that positive feedback from colleagues, friends, teachers, etc. does not compensate for the responses from their parents which are/were lacking? I basically agree with Dr. Wolf's response (though CT would express it a bit differently) but there some additional parts to the answer as well.
First, imagine that I know people who seems to think that I am OK and I know people who seem to think I am not OK. Who do I believe? If the people who know me best (i.e. my parents) seem to think I am not OK and the people who know me less well (teachers, bosses, friends, ...) seem to think I am not OK, I may well believe the people who know me best and positive feedback from others may not be accepted.
Second, humans are prone to what academics call the Confirmatory Bias. Once we form a preconception, we tend to look for evidence which supports our preconceptions and we tend to overlook or discount experiences which should contradict our preconceptions. Once Gary had a view of himself as incompetent, he tended to focus on experiences which seemed to show that he was incompetent and tended to overlook or discount experiences which showed his competence. This perpetuated his dysfunctional belief but it isn't something which is specific to psychopathology, this phenomenon is well documented in a wide range of normal populations.
Third, people often cope with their doubts, fears, and insecurities by maintaining a facade. If I am doing my best to maintain a facade that I am competent or nice, or likeable, or whatever and then I receive positive feedback from others, it is easy for me to believe that the positive feedback is due to the facade that I maintain and that people would react very differently if they saw me as I am.
Finally, if I accept what I can get from contemporary relationships and stop striving to attain whatever I seek from my parents, that means accepting that I may never get what I wanted from my parents. If I do this, I will probably experience feelings of sadness, loss, and helplessness. Individuals who avoid aversive emotions may refuse to abandon their efforts to attain the desired response from their parents because they are unwilling to tolerate these emotions.
What does this say in terms of intervention? "The repair of disrupted relationships with the therapist" may be one way to remedy this situation. Another option would be (1) to establish an accepting, collaborative relationship with the client, (2) to work with the client to counter the biases which make it hard for them to accept positive feedback from contemporary relationships, (3) to help them to form a realistic view of themselves, (4) to help them to gradually relate to others without maintaining their usual facade, and (5) help them tolerate (and cope with) the emotions they experience in the course of doing this. If "disruptions" occur in our relationship with the client, we will need to repair them but we do not see this as an essential part of treatment.