I have copied the original statement here for easy reference. 

 Suppose you're on a game show, and you're given the choice of three doors: Behind one door is a car; behind the others, goats. You pick a door, say No. 1, and the host, who knows what's behind the doors, opens another door, say No. 3, which has a goat. He then says to you, "Do you want to pick door No. 2?" Is it to your advantage to switch your choice? 

 So what are the possible outcomes?  We cannot even begin to answer that question without making assumptions about things that are not explicitly stated.  For example, it says, You pick a door, say No. 1.  It does not explicitly say that you might have picked door 2 or door 3.  This is not an unnatural assumption considering it is a game show but it is an assumption. 

 The same is true of the host's choice of door. Must he always open door 2? 

 Now I agree that these two particular questions are answered by the 'standard' rules of the game that are assumed by most people and that we have agreed to assume here but I mention them to show that, right from the very start, we must make some assumptions about what is important and what is not when we propose a sample space. 

 The host says "Do you want to pick door No. 2?" We are not told that the host might have used different wording but logic tells us that normal speech often uses different words to mean the same thing.  Maybe the host sometimes says "Do you want to switch to door No. 2?"  The standard rules tell us nothing about the host's choice of words although we are clearly told of one possible wording in the problem statement.  Our sample space should therefore include every possible form or words that the host might have used, just as it includes the doors that the player might have originally chosen. Why do we not do this?  Because we have to decide which are the important events before we set up our probability space.  There is no procedure or algorithm that tells us how to do this; it is a matter of interpretation.