True story...
A few years ago, I had my then 16 year old son on the back of my Wing and we were going home from a restaurant. It was his first time on the back of a bike since he was probably 6 years old and he was loving it. I was looking in my mirror and I saw that he stuck his head out from behind mine to look at what was going on. I saw that he had the biggest smile on his face and then I heard him shout: "Ewwww!" I said: "What's wrong?" He shouted back: "I just slobbered all over myself!"
In that one moment I realized that not only do I know why dogs stick their heads out the window, but I also know what they say when they do it.