Compassion

Not that Traveling Doctor Liu minded. 

"A doctor has great compassion, but it didn't mean that we're full of kindness," Traveling Doctor Liu. "A doctor is also a human being with desire and selfishness. I can't save everyone in the world with my skills and it's something I have known for a long time."

Traveling Doctor Liu didn't appear out of a rock. He also has a master and also learned many things when he was young. His master was the one who taught him that while doctor had to be full of benevolence, there was a limit to it.

Because they were not a saint who could save everyone in the world from suffering.

They were just a doctor, a profession that humans made. 

He learned medicine to save people but it didn't mean that he would be able to save everyone. 

Who could be saved and couldn't be saved…

It was the first lesson his master had given him when he went out to do his practice for the first time back then.