Not really a word, but it should be. "Sonder."

I used to get this all the time out in crowds, or driving on the freeway.  Hat/tip ashlllleigh 

Is sonder a real English word (the realization of the richness of others' lives)? - Quora: "Is sonder a real English word (the realization of the richness of others' lives)?"

sonder - Wiktionary: "Coined in 2012 by John Koenig, whose project, The Dictionary of Obscure Sorrows, aims to come up with new words for emotions that currently lack words.[1][2] Related to German sonder (“special”) and French sonder (“to probe”).[3]"