I will often acknowledge other cyclist/dog walkers/country lane murderers with a hello or head nod if they feel the need to interact.
However my most of my cycling is in the form of an 11 mile commute often after a 13 hour night shift and sometimes I just wanna get home to bed, I don't want an assumption of my personality made on whether or not I said 'hello'
I dont feel the need to drive around with my head out the car window waving at every other motorist or talking to every bloke who has the same shirt as me.