Shaking My Head

I find myself shaking my head a great deal.
Why does one shake their head in a motion that connotates disapproval or negation?
For me, it is when someone has perpetrated an act that I consider stupid or blatantly in the face of common sense.
We could easily devolve into an argument of what common sense is, but we shant.
Today, I was driving home from work, in a good mood from a full and successful day, when a large truck, 4 door and bigger than necessary, turned out of a strip mall and landed in the two way turn lane. He accelerated but not up to speed to pass me or merge safely, but he started to enter my lane with the bumper of his truck even with my front tire. I took my foot off the gas and decelerated a bit so he could safely take over my lane of travel.
I shook my head and restrained myself from making a rude gesture or honking my horn, I just waved at the driver of the too big truck as if to say "Hey, I am here".
Maybe he did not see my red truck. Maybe he was thinking too far ahead of himself and was not aware of his now, his present.
I shake my head so often when I drive that I have relieved much of my whiplash injuries. I actually have a stronger neck because of it.
I look at these people so distracted, so not in the moment and focused here that it is truly a wonder more people are not injured or killed in automobile accidents.

Just do me a favor.
Stop!
Pay attention.
Breathe.
Focus on the moment you are in right now, so I do not have to shake my head at you.

C