Dogs are always conscious of their status in any pack hierarchy. And while a dog accepts his position in the pack, he is, to his way of thinking, upwardly mobile.

the dog notices, in the car, that the seats are status ranked. The more important you are, the farther front you sit. If there is only one person in the car, that person sits in the driver's seat.

Dogs notice this. If a seat of higher status is vacant, the dog is only too happy to move into the slot. That's why the dog always wants to drive.