The simple fact is that traffic stops are a lot of things, but routine is not one of them. Clearly, there are a whole lot of things that can go wrong during one.
The traffic officer has got to be alert for offenses going on around him, especially those that could cause injury and/or property damage. Before turning on his lights to make the stop, he has to try to do so in an area and way that will not endanger other motorists. Upon approaching the violator, he has to determine what danger might await him from the violator or others in the car, and also decide what he is going to do about the offense. Will he issue a ticket, a written warning or just a verbal warning?
Finally, it is critically important for the officer to understand that, while he is only stopping the person for a traffic violation, in the violator's mind he is being stopped for the worst thing he's ever done. The person who is only being stopped for failing to come to a complete stop at a stop sign may, in fact, be wanted for armed robbery, murder or any number of serious crimes. And, as in the case of the Virginia Tech officer, a person totally uninvolved in the traffic violation may just walk up off the street and start shooting.
I encourage all police officers to be as polite and friendly as the situation will allow them to be. However, citizens should try to understand when the officer doesn't seem to be so polite. The police officer making that "routine" traffic stop has got a lot on his mind.