Ever think about the thousands of people you pass by as you drive down the street? Ever wonder, as you take a glimpse into their lives, how they feel at that moment? Maybe even what they could be going through in their life? But what does it matter? To you, their lives mean nothing. They don't concern you. To them, their lives are everything, the only world they know.
You've perfectly described the word 'sonder' - the profound feeling of realizing that everyone, including strangers passed in the street, has a life as complex as one's own, which they are constantly living despite one's personal lack of awareness of it.
Yes! This has occurred to me multiple times! What's funny about this realization is that it can, at different times, be realized slightly differently, and those subtleties can make me either uplifted or depressed.