stinkynuts does have it right with the analogy
When you first pick you have a 1/3 chance of winning. I think most would agree with that.
But this means that there is a 2/3 chance the automobile is behind the other doors.
When he shows you what is behind one of those doors the odds do not change, there is still a 2/3 chance that the automobile is behind one of those doors. Of course he knows which door to show you, the one that has the goat. That means there is now a 2/3 chance the automobile is behind the other door you did not pick.
Therefore you should always switch.