In "Albatross", Spock breaks McCoy out of prison to find a cure for a disease which is killing the Enterprise crew. The Vulcan reminds him that if he goes and fails, he will also die. Bones' response? "I'm a doctor, Spock. A doctor. Get us beamed aboard!" Not since "The Empath" has one of McCoy's reminders about his profession been so heart-meltingly awesome.
As is his joy and relief when he finally gets proof that he didn't inadvertantly cause the previous plague.
In "The Pirates of Orion", McCoy's attitude around the terminally ill Spock.
Thelin: True. A warrior race has few sympathies, but one we do possess is for family. In your time plane, you will live and so will your mother. That is valuable. [gives Spock the Vulcan salute] Live long and prosper in your world, Commander Spock.
Spock:[returning the gesture] And you in yours, Commander Thelin.