when I re-started serious riding 2 years ago (BTW serious cycling is rying to fit in 2K miles/year with a high stress job) I was amazed at the number of fellow roadies who ignored my 'wave/greeting" But I find myself now doing the same. Why? I think its because I am so focused on what I am doing that I just don't notice much else. I focus on my body, my mind, my thoughts (great stress reliever, and have solved many work problems on the road) to the point that I ignore everything else -except traffic.

I often pass someone who waved before I have a chance to wave back.

BTW- I'm a 'lone wolf' rider and like it that way.