The boundaries between the sane and the insane are not as as clearly delimited as most people would like to believe. Many times, people's capacity for impulse control and keeping the appearance of normality make the difference between being labeled mentally ill and just being stressed out or overwhelmed.
The term insane has been traditionally used to ascribe negative value and condemn that which is different from established social norms. Established social norms are not monolithic. They vary depending on the place and evolve throughout history.