Diagnosis blocks!
To start with, I certainly try to understand the person from diagnostic point of view.
Once I decide to work with him / her, I keep aside my diagnosis and become a co-traveler in my client's journey of self-search.
I have made this decision on the basis of common observation that diagnostician becomes a superior person and the person becomes a case under microscope. This trend produces 'I and you', 'Me superior you inferior', 'I know everything and you know nothing' kind of non-verbal message. This can produce a block in the process of building therapist-client relationship and thus hinders the work.