Back when I was an instructor in my clubs' Road Skills course, one of my students commented that I was the friendliest biker he ever saw, because I always wave at drivers at intersections.
I explained to him that I wasn't doing it to be friendly, I was doing it to make sure I had their attention.
But over time, I realized that I like to have that little bit of contact with the drivers around me - to acknowledge their co-existence with me on the road, to give them a little "Thank you for not killing me" wave as I pass by them at an intersection or when they give me the right-of-way.
And I think other drivers appreciate it too.
2009 Lynskey R230 Houseblend - Brooks Team Pro
2007 Rivendell Bleriot - Rivet Pearl