Through common sense, and process of logic.
But I will spell it out for you.
The transferring of knowledge doesn’t need to be shared on both sides.
For example, a journalist can write a story about something they don’t agree with or personalize with.
A therapist, even if they’re inherently sad, can still, and arguably have, even more ground to relate to the patient. Therapy is a study, it’s a process of easing someone, not sharing emotions like some sort of hive mind.