Each of the doors has a 1/3 chance of having the car. Thus, whichever door you pick has a 2/3 chance of being a goat. When the goat door is opened, it doesn't change the probability of your door having a goat. So you have the door you picked, with a 2/3 chance of being a goat, and the door you didn't pick, with a 2/3 chance of being the car. Switching makes the most sense.