A juvenile loser with nothing else to do but to run into a gas station and grab a bunch of stuff, then run out the door screaming Yahoo! The person behind the counter then screams out Hey you! in an attempt to call you back into the store

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.