SONDER

Sonder
n. the realization that each random passerby is living a life as vivid and complex as your own-populated with their own ambitions, friends, routines, and worries -an epic story that continues invisibly around you, in which you might appear only once, as an extra sipping coffee in the background, as a blur of traffic passing on the highway, as a lighted window at dusk.

Source: The Dictionary of Obscure Sorrows