For a while there, I used to shout out a friendly 'Hi!' whenever I encountered another runner.
But after getting the silent treatment in return one too many times, I stopped. But then I felt awkward whenever I saw another runner. Worrying about things like: Should I pretend I didn't see him or her? Avert my eyes? Look down to the ground? Suddenly develop an interest in my smart phone? Pretend I get an important call?
Then I decided: 'Sod it!' I'll do the right thing and greet people. If they can't return the favor, well, that's théir problem. At least I've shown them common courtesey.
Do you greet other runners?