I am a courteous driver and I appreciate other courteous drivers. Really.
However, I do not appreciate "over-courteous" drivers. These specimens can be difficult to discern from a similar group (and may in fact be the same group) that we know as "drivers with their heads stuck up their anatomy". I don't dislike these people but I dislike their driving and wish they would understand why they are dangerous.
You might see these people yielding to other cars when they in fact have the right of way. That is noble and I encourage it when you are leaving the jammed parking lot of a Thin Lizzy concert*. However, when in the flow of moving traffic, many nearby drivers will be operating under the assumption that traffic rules will be obeyed, those with the right of way will proceed, and those required to yield will do so. When one driver decides to change the rules out of politeness, it is very disruptive to all other drivers who expect traffic rules to be followed. This even includes the recipient of the polite gesture.
Because these courteous drivers interrupt the expected flow of traffic, they in fact drive unpredictably, which is to say "dangerously".
* Thin Lizzy is still drawing the hip crowd, dog.