Cops have a tough job. I'm friends with one, and no, criminals don't wear signs on their foreheads. A person doesn't have to have a criminal background to do something. The first offense is the... first... That, and peoples' brains tend to go out the window when they're panicking.
People have tried to run over stupid things like an expired license. When he pulls someone over, or gets involved in a situation, he never knows what he's dealing with. He could be pulling over someone with a concealed weapon and an arrest warrant. That person could panic and try to avoid arrest by shooting him.
The cop in that video was a bit aggressive from what I watched, but they do have to be careful and stay alert. Like Oniya said, keep your hands visible and don't make any sudden moves. It may be natural to be nervous when getting pulled over, but they pick up on that and it makes them nervous too. They don't know whether or not you have a good reason to be nervous, such as drugs in the car. Over time they learn to tell, but they'll still watch you.