My best guess is, that as humans our brains are especially sensitive to the facial expressions of other people. And, a shocked expression is an incredibly impor

My best guess is, that as humans our brains are especially sensitive to the facial expressions of other people. And, a shocked expression is an incredibly important signal. Looking at someone wide-eyed with an open mouth in front of you implies there is something you really need to pay attention to. Just watch how babies react when you make the same face. I suspect social media just hijacked and metastasized the response.