How did the two couples become friends?

The couples became friends aboard ship while traveling. They gravitated toward each other based upon their shared values and principles.

Between the Macphails and the Davidsons, who were missionaries, there had arisen the intimacy of shipboard, which is due to propinquity rather than to any community of taste. Their chief tie was the disapproval they shared of the men who spent their days and nights in the smoking-room playing poker or bridge and drinking.