Can you explain variable change?

I'd heard about it before with the example used in the movie 21, but I still don't really understand this mathematical concept. why doesn't the probability reset after a wrong choice is revealed?

here was the scenario given:

The professor tells the main character that he is on a game-show and the game-show host tells you he has 3 doors: Door 1, Door 2 and Door 3. Behind one of the doors is a sportscar. Behind the other two doors there are goats and he asks you to pick a door at random. The person chooses Door 1. The game-show host then opens Door 3 and reveals that is has a goat in it. He then asks if they want to keep their initial answer of Door 1, or switch their choice to Door 2?

i understand that with the theory he should change to door 2, because there was a 66.6% chance that it was either door 2 or 3 and a 33.3% chance that it was door 1. So if we know that it's not door 3, then it becomes 66.6% for door 2 and 33.3% for door 1 so we should switch.

what i don't understand, is why the probabilities don't "reset" after 1 of the 3 variables is revealed?

thanks,

jeff