Here's how I got there. To an individual in therapy, the psychologist is their refuge. An escape to truth from the purgatory that is their life. The one person they're honest with. Their priest. The one hour a week they feel free.
How could they not fall in love with this person?
Aside from the obvious restraint and oath preventing reciprocation of this love by the therapist, it has to be happening on a comparatively massive scale, no?