I doubt they are "trained to act serious" but there is a culture in the Force of that. Everybody's got an idea of how a cop, doctor, construction worker, etc "ought" to behave.

For him, its just business. You've also got to keep in mind that whenever they interact with a citizen in a country where guns are as common as they are in the US, they always keep a level of concern regarding their safety. They never know just who they're going to stop or what they're going to find . . . Maybe the guy in the 911 Turbo is a drug dealer with a crack pipe in the passenger seat and an automatic weapon in his hand . . .