You've probably seen the Youtube Face; it's that extreme facial expression (disgust, ecstasy, hilarity, etc) depicted in the thumbnails of Youtubers' would-be-viral videos, especially reaction videos.
Joe Viex traces the origins of Youtube Face and its accompanying memes (like the singsongy Youtube Voice, "Hey guys! BlackheadDigger420 here, sorry I haven’t posted in a while…"), from "a mix of ~2012 Facebook newsfeed viral garbage with generic chumbox aesthetics" to the current visual vocabulary, part of an attentional arms-race in which Youtubers compete to please the "Algorithm Gods" and claim some advertising dollars.
It's all part of the great reversal of "brands," in which "People simulate the behavior of corporate brands, while corporate brands simulate people."
At some point, a user discovered that a catchy preview image tended to trigger potential viewers’ curiosity enough that they clicked through more frequently. Most likely this notion was inspired by other forms of clickbait (in style, it seems to be a mix of ~2012 Facebook newsfeed viral garbage with generic chumbox aesthetics). Then another user discovered that including a facial reaction tended to boost views further (perhaps manipulating some kind of primal feeling of empathy or morbid curiosity in the pain of others?). Over time, view count metrics gradually pushed these facial reactions into more exaggerated expressions, making everyone look like extras in a Soundgarden music video.
The aesthetic seems to have been largely popularized by the “reaction video” genre. There’s The Fine Bros, who make videos of old people reacting to new things, and young people reacting to old things. Their ~1,500 videos have received over six billion views.
Your Pretty Face is Going to Sell [Joe Veix/Openspace]