I am not being defiant, this saying does exist and makes sense
Doctors make the worst patients.
It’s difficult to advice a person on a matter in which he is an expert. This is because he thinks that he already knows the best about what to do.
Example: When I tried to advice my friend on how to increase revenue from his gas station, he argued why my suggestions wouldn’t work. He was clearly jumping the gun and not open to ideas. Doctors make the worst patients.