Contrary to the impression perhaps given by the photograph, it is not because he is about to be shot. He being me, the answer might be "because he never actually smiles" although of course I rarely do.
It is because I have tendonitis, and bloody painful it is too.
You might reasonably ask "what's that got to do with chess", but no sooner had I posted the image on Facebook then a friend asked Is it your chess playing arm?
It's not, since I mostly use the other one, not that I've used either of them in a real game since last August. Still, at least I can identify which arm is my chess playing one, which may be more than one can say for Hikaru Nakamura.
What would I do if it was my chess arm? I imagine I could use the left one easily enough to move the pieces, but writing down the moves might be a larger problem. The Laws are clear: not only am I obliged to keep score, but if I cannot do so in the normal way I have to supply my own assistant to do it instead.
In vain would I argue that nobody can read my scoresheets even when I complete them with the right hand.
Anyway, I was idly Googling away to find other examples of tenuous links between tendonitis and chess and who should I come across but your friend and mine?
You have to strain a bit to find the reason for the title, which turns out to be that
for all the impressive statistics, he (Magnus Carlsen - ejh) does have one Achilles heel, namely a certain vulnerability when facing White’s most aggressive first move, 1 e4.This assertion turns out to rest on the game Carlsen lost to Adams - which may loom larger in Ray's mind than it ought to because he's used the same notes to that game six times - and a blitz game he lost to Svidler in 2006. This is thinner than one would normally associate with Ray, and come to that, while it is true that there is such a thing as the Achilles tendon, it wasn't the tendon which was Achilles' downfall.
I had no idea the story wasn't in the Iliad. See, it may be Monday morning but you've learned something new already.
And my bloody arm hurts.