Mary Me wrote:

May I present you the Napoleon of crime and the world's only consulting detective fighting each other like true men. 
In which Jim takes Sherlock's chair only to piss him off.

 
You can see it the other way round, too.
Sherlock knows that Moriarty prefers to have a sniper with him. According to canon Moran is the second in command and Holmes knew it.
Conclusion: Sherlock wants to see through the window and he doesn't want to sit in his own chair with his back towards the sniper.
What can be done to get Moriarty to sit there?
You know that he is proud of being changeable, of doing the unpredictable. He said that at the pool.
He doesn't want to be manipulated of course.
Conclusion: You give him the chance to have a point, to win on a real small scale and do the exact opposit of the expected.

I'd love to believe that Sherlock was secretly protecting Jawn's chair :3

besleybean wrote:

Huh?

You know how both Sherlock and John seem to have their own chairs? John's is the one that usually has the Union Jack pillow? Well, I know it's not true, because it's far too sentimental for him, but I'd like to think that Sherlock tricked Moriarty into taking his chair so that he would not choose to sit in John's.

I happened to be unhappy with Sherlock when he offered up John's chair. I wouldn't like my best friend's worst enemy who tried to kill us in my chair. It's extremely sentimental and not practical, and I suppose the explination could simply be that Sherlock wished to sit in his chair, so he offered the most conviniant alternative to Moriarty.