This is an old video that is circulating everywhere right now, and the reason why is obvious: people need a laugh. Sometimes they don't even know they need it until they're trapped in a subway car with a lady who is emitting some kind of laughter-inducing pheromone. In the video below, everyone slowly succumbs to the mass hysteria, no matter how slab-faced they started out. And you will, too.
Don't try to resist! You know you want to join! The subway is a terrible place, so if you have the chance to laugh about something, take it. Who cares what's on this lady's phone? Hopefully it's not something horrible, like a dead dog, but...oh no. Bummed myself out again.