If the question is, that the car is behind which door (from the remaining two), then the probability is 50-50.
BUT, the question here is: Do you have more chance to win if you switch or if you stay with your first choice? In this case switching means 66.6 and staying 33.3.
So you are both right at some point. That's why it is a paradox.
I think this pic helps to understand.
