First of all, Callahan is completely right- THERE IS NO SUCH THING AS A 'ROUTINE' TRAFFIC STOP. There are no 'low-risk' traffic stops, there are only high-risk and unknown-risk stops. Every time I stop a car for a traffic infraction, I have no idea who (or what) is in the car. I've had polite, respectful treatment from hardcore felons who had felony warrants, and I've had to fight with Joe Businessman who had a bad day and thought I was picking on him for giving him a speeding ticket. I've had normal-looking citizens try to run me over on a traffic stop. Other officers have had well-dressed, professional-looking women pull guns and try to kill them just because they were tired of getting tickets.
Here's the problem. When you, Joe Citizen, get pulled over for speeding or whatever, you know that you are just a normal guy who happened to make a mistake. You know that you don't have any weapons or drugs in the car, and would never in your wildest dreams think of assaulting the officer. The difficulty comes when the officer doesn't treat you like he knows these things, too. You assume that the officer should intuitively know that you are 'one of the good guys'. Frequently, people get offended and think the officer is being rude when really all they are doing is being firm. Most people don't like being told what to do, no matter how professionally it is done, and will perceive this as rudeness on the officer's part.
It's true that some officers have difficulty being firm but polite and remaining in control of an encounter without coming across as overly aggressive or badge-heavy. This is a skill that is learned only with experience. Some officers don't have good people skills and never learn how to be truly professional. That's life, folks. We don't have a giant pool of perfect people from which to draw police recruits. In fact, the quality of recruits seems to be going down nationwide, for a number of reasons that I won't get into here.