Okay, so I work in a building with about 3000 other people. Every day I see people I've never seen before walking down the hall. I always make an effort to at least smile at the other person. Most of the time they smile back, and if they look pleasant enough I might even offer a verbal greeting.
What I don't understand, though, are the people who, when I smile at them, either stare straight ahead as if I don't exist, completely ignoring me, or look at me like I have a third eye in the middle of my forehead. What is with people? Yes, I've never seen you before and may never see you again, but would it really hurt that much to smile?
Of course, most of the people here are engineers (yes, I'm Dilbert), so they are not known for their social skills. But come on! At least make an effort!