We are not solitary animals; we evolved to be part of tribes and bands. There's a part of us that's just not happy unless there's other humans around.

And it's perfectly cool if we never interact with them. That's fine. Maybe sometimes all we want is to have someone around to be faster than when a lion comes out of the shadows and takes the slowest person down. Even if that, or some metaphorical equivalent of it, is exceedingly unlikely in a coffee shop.