When laughter itself is the transgression, the videos pack a one-two punch. It’s not normal for a woman to laugh to herself at length in a crowded subway car, but it’s harmless, so as her laughing fit continues, others can’t help joining her. A similar thing happens when performers on “Saturday Night Live” break character and laugh, as Aidy Bryant and Vanessa Bayer did in a sketch last week featuring Fred Armisen: The audience laughs, too.

Online videos feel particularly intimate because they largely feature amateurs, are unscripted and are filmed up close and by hand. Ms. Payne “is a pleasant and attractive person to whom we freely lend our sympathies,” said Carl Plantinga, a professor of film and media at Calvin College who studies the psychology of cinema. “Her enthusiasm seems to be genuine and kind.”

And because she’s filming herself in close-up, her emotions are front and center — even once she puts on the mask, you can see her eyes glistening in delight. “Sympathetic viewers will involuntarily mimic her facial expressions, leading to emotional contagion,” Mr. Plantinga said. The use of Facebook Live — which streams live video instantly, with no opportunity to edit or scratch a take — only draws the viewer in closer.