Acknowledgement

Yesterday as I left a high school after subbing for a day, one of the students I had called across about 25 feet to day goodbye to me, by name. This really surprised me, because while I knew his name (by chance cause remembering names is not my strength), I had him for just one period. However, this lifted my mood for the rest of the day and even causes me to smile a bit writing it today.
If simply saying "Bye Miss ..." can mean so much, why don't we acknowledge others as often as we could? One of the reasons I chose to go to a medium sized school was so I could see people I know when I walk from one part of campus to the other. It's not as small as high school where I knew everyone, but seeing someone you know and actually acknowledging each other is a mood lifter. That two second exchange, which doesn't always require words, reminds people that we are thinking of them and that they are important. It shows that we know them and care about them.
As I walked into the library last night for class, someone commented "I don't see you much around campus anymore." Sadly, that's true. Even though I spend 26 hours on campus between work and class, plus friends/clubs outside of it, 23 of the hours at least are spent in one building...mostly in my office. I kinda hide in my office, but whenever I see someone walk by my office that I know it's exciting to be able to acknowledge people.
When it is so quick and easy to smile at someone, to say hello, or to even just nod, why do we sometimes quickly pass them by without even acknowledging them? For your good and mine, let's take that 2 seconds to transmit the fact that we know someone and care about them.