You probably didn't even know there were rules for calling "Shotgun!" but there are, and people who don't know them shouldn't even be allowed ro ride with other people. Here's the most important one:
Shotgun may only be called after the "deed is done". Simply stated, the deed is any activity or objective that directly precedes the ride in the automobile. The deed can be anything ranging from a visit at a friend's house, to a shopping trip at the mall, to a visit to the Grand Canyon. We cannot stress how important this is because this establishes a Shotgun-calling time frame that ensures everyone has an equal chance of recognizing when to call Shotgun.
There is no crime greater than calling Shotgun on Monday in reference to the ride to the concert on Friday. Some people choose to play this way, and they are fools.
The site explains that the history of Shotgun! goes back to covered wagons and the Wild West. I think the writer really meant stagecoaches rather than covered wagons, but never mind that. The interesting point is that riding shotgun was probably considered dangerous, and so is the modern version. Why do we all want to sit in the one seat in which we're mostly likely to be killed in a crash?
