Driving Criticism

I'm sure I'm not the first person to consider this (but hey I'm not an original thinker) but why are we so defensive and touchy about our driving? You can't criticise someone else's car control or gear changing without risking a diplomatic incident bigger than Russia trying to put missiles on Cuba. Why is this? Why do we all think that we are the best drivers in the world and any doubt expressed by anyone is equivalent to having one parentage questioned?
I am a honest person. That should not be mistaken for being rude. I try to avoid rudeness but I will try and be truthful of my opinion is sought, yet I cannot bring myself to say nasty things about the driving of others, however useful it maybe to them or the structural integrity of their car.
It is quite possible that this has become the last taboo. You can discuss anything you like in public, a naked man can stand on the forth plinth (despite the protest of some spoilsport who can't tell the difference between the naked form as art and porn), BBC 2 can show a programme that uses the words “Nigger” and “cunt” quite a lot but try to tell someone that perhaps they should practice parking a bit more and they will try to have you tried as a witch.
There is, of course, only one time when it is acceptable to diss the driving of others and that is when you are behind the wheel. “look at this twat” “go on, you could get a bus through that gap” etc seem to flow from those who would either cry or shoot you in the face for saying the same thing about them when they slipped on the comfortable driving gloves.
I have no answers to this. It just strikes me as slightly odd as I sit next to significant other thinking that she really should change down but being to scared to say anything, even though I am the person who made her cry whilst she was looking for some jeans (I'm not proud of it but I did find her a really nice pair afterwards).