I also like saying hello to pedestrians I have stopped to let cross (I do that a lot, when safe to do so), and drivers of convertibles who have their roofs down.
I guess I'm just a friendly person.
It's one of the great things about cycling - how convivial it is.
I once asked a guy in an SUV if any of the emails he was reading on his phone were interesting as he trundled along in the traffic I was filtering past. He jumped like he'd been stung.
I say 'good morning' to the lollipop peeps when I pass them each morning, although I don't always commute the same route so haven't seem them since the schools went back. One says 'good morning' back, so that's nice.
Near the beginning of my commuting I was at the bottom of Bluebell Hill, about to do the ascent and a guy coming down saluted me, which near made my day. I still almost died from the exertion, but I'd have done it with a confusing smile on my face (Phil Liggett would then have reminded everyone it was not a smile of joy, but pain).
I always try to post something in a thread about waving to people, I think it's equally polite :-)