You need to know that the host isn't acting malevolently. If he is, your probability is zero if offered the opportunity to switch. If he isn't, your probability is .67 by switching. That is, there is a factor that the probability is conditional upon - the host's intentions.
That is, if there were 100 doors, and one prize of a million dollars, after picking a door, the host could say, "I'm sorry, you lost." Or, the first and only time the game is played, with the host knowing which door is the winning door, upon realizing that you picked the winning door, the host could show you 98 of the 99 losing doors, and then ask if you want to switch to the last door remaining. Dumb people might get lucky by not knowing the Monty Hall problem and having poor statistical skills. But, smug smart people would switch every time, because no one explained to them that the host could be acting malevolently. Monty Hall offered the opportunity every time, because this made his show more exciting.