“All else being equal, when we’re alone, our brain is a little more vigilant for any signs of danger. Also, our brain perceives demands from the world as more demanding than they would be if we had someone with us,” UVA psychology professor James Coan said. “And there’s a really simple reason for it: It’s that the world is more demanding when we’re alone, because anything that the world demands of us when we’re alone, we have to do by ourselves.”