I agree that training effects approach, but I also think that the therapist or psychiatrist's personality and personal style also very much comes into play. My current pdoc just happens to be extremely warm, open, funny and shares a lot and I feel very comfortable and safe with her. Same with my LCSW T. They both have very flexible boundaries.

It could also be that a T picks up on what will work for an individual, boundary-wise and works with that, keeping in mind what they are comfortable with themselves.