So many people, while walking, keep their heads facing the sidewalk. Not just girls in high heels trying not to misstep on the concrete. Either there is something incredibly interesting about the sidewalk I haven't yet discovered, or people just don't realize that the view is much better at eye-level.
You get a chance to make a connection with someone, if only for an instant, and you can practically change the course of someone's day with a simple glance and a few muscle movements. It doesn't even have to be a smile. You don't have to show any teeth. You can pull one of those, "I acknowledge your presence" nods that guys like to give. Unless you know the person - then by all means, do more. Dare to say hello! But the opportunity to push a little of your positive energy in someone else's direction is presented so many times throughout the day. (Hermits excepted). And so often it goes unanswered.