Well speaking from experience, one beer leads to another and then one more and then just one more and so on. When I drank I really had a hard time stopping until I either went to bed or passed out. I always admired my friends who could just stop or limit their drinking. Alcohol affects some of us differently than others so I really don't try to make sense of why someone acts they way they do when drinking. Kind of like arguing with your wife, some of us know when to shut up and some of us don't.
...and regarding your point about the 1 car, one thing that does get to me is when one driver ruins another driver or drivers day and then just proceeds to carry on their merry way.