The Uncomfortable Passing

So, I'm walking down the street to the traino and a girl, about my age, maybe a little older, maybe a little younger, approaches me traveling in the opposite direction. Nobody else is around, just her and me, headed for an awkward collision. My first reaction, after the initial realisation that there is no other option but to pass her, is to get my head down. Maybe I will be able to pass, by pretending I never saw her coming? No, that's certainly not going to work! What do I do? She's so close now. Ok Chris, come on, lift your eyes, say something as she passes. Acknowledge her. Smile. Walk on by. Ok, here she is, we look up, our eyes meet, she bows her head and walks past without even a nod of the head or a smile! Boy was that awkward! 

Why does this always happen when I pass someone my age? Is there some secret code which states that a young person should never acknowledge the passing by of another? When you pass an older person it's easy to look up and say "good morning" or an appropriate alternative for the current time of day. When you pass someone younger it hardly matters, they might give you an inquisitive smile or comment, and you the same. But when it comes to someone your age, it's always the "keep your head down until you pass awkwardly", and continue on to your respective destinations.

What is so hard about saying hello to your respective teenager? Why is it so difficult to spare a little smile as you wander by? Until I find a stranger that can acknowledge my passing, and I theirs, it will continue to remain a mystery...