Yoshimoto Ishin was a businessman and devout Jodo Shinshu Buddhist who, as a young man, had engaged in an ascetic contrition (mishirabe) practice involving sensory deprivation, by dwelling in a dark cave without food, water or sleep.
Wishing to make such introspection available to others he developed Naikan as a less difficult method which he first introduced to young people who had been incarcerated for committing crime and social disturbances.
More contemporary proponents included Albert Schweitzer, Ben Franklin, and Bishop Fulton J. Sheen.
And third, the application of questions or structure which helps us examine our lives with an emphasis on our conduct in relation to other people, creatures and objects.
However, as with Buddhist metta meditation (mettā bhāvanā), there is no reason why Naikan practice need necessarily take family relationships as starting point.
The benefit of looking at family relationships is that these are often most emotionally complex and connected with our sense of 'self' [citation needed].