Kirk assumed it was Spock, not Barrows, that was giving him the backrub. He would have preferred Spock.
Concerned about Kirk's health, Spock cleverly convinces Jim to order himself to take shore leave.
Concerned about Jim's safety, Spock disobeys orders to beam down and be with him.
Spock knows Jim's real reason for fighting with Finnegan even before Jim does.
Through the whole episode, all Spock seems to care about is Jim's health and happiness.
Spock rejects a beautiful "woman", preferring to ship-sit, so Kirk can relax and have some fun.
He doesn't understand all the crazy things that Kirk likes to do, but he accepts it because... well... he just does!