pronouns - "that" or "this"

I know a trivial difference of "that" and "this" uses. Such as the difference between "this chair" and "that chair".
But I don't understand one case. When one person is coming into a room and another has been doing something. And the first one asks him "Why are you doing that?". I heard about the same questions with "that" in many analogous situations in different movies. And the main question is why "that", not "this". I think that "this" more fits here, because a situation is in front of an asking person.

Answer

Although the action may be taking place in front of the person asking the question, it is nevertheless taking place at a distance, both physically and situationally. It's the OTHER person who is performing the action, and so it's taking place 'over there', in a different personal space. Why are you doing this, on the other hand, would often refer to an action that affects both parties.