Therapist should choose patients carefully.
Human mind is biased. Realizing and accepting that fact is first step in becoming a good psychologist.
If patients problems are reminding you of your past or making you emotional, you should not deal the patient. If you deal, therapy becomes biased. You see patients problem from your own perspective, but not from their perspective.
If your patients relative joins therapy, you have to send him/her to other therapist. If you deal with two parties, your therapy will be biased towards one party because of other party.
As i mentioned in earlier post, psycho dynamic therapist should be a stranger to patient.