In the ‘Great Game’ the laser sniper is initially directed at John’s heart:
And at Sherlock’s head:
Just right after ‘the ripping of the clothes part’ when they laugh & then Moriarty turns up, both of the sniper lasers are directed at their hearts.
Meaning one thing only, in my opinion:
At the beginning Moriarty considered John ‘The Heart’ and Sherlock ‘The Head’. Of course he did so - after all, Watson was the ‘so loyal pet’ of the great Holmes. John was the one expressing his astonishment by the genius Sherlock is. They became such involuntarily. Cause till this point neither of them has ever considered the fact they were leaving in half. John needed someone to sort out his thoughts, to feed his mind with the thrill he has always longed for. Sherlock, on the other hand, has always lived in solititude where sometimes his brilliant deductions were just not enough. John needed ‘The Mind’ or, anatomically speaking, ‘The Head’; Sherlock needed ‘The Emotions’ or ‘The Heart’. That’s why the first time they met Sherlock already knew they were right for each other - he used his head to make this deduction. John also knew this was the beginning of something very powerful - he sensed it in his heart.
But, as it happens most of the time, people do not realise what is going on in their lives until someone else points it out to them. It’s Moriarty here. Although a perfectly trained soldier, John expresses his emotions - his legs give up right after Moriarty walks out of the room. Although the world’s only consulting detective, Sherlock’s mind also gives up a bit - he stumbles while expressing his gratitude to John. And right at this very moment - when they both realise they are really one person in two bodies, they start laughing. A laugh expressing their relationship from now on - full with thrill, chase and adventures; but also full with fun, joy, and emotions. Moriarty does realise this too and just points it out to them, by directing the laser sniper at Sherlock’s heart and neck too. The heart and the neck - the specific points where one can feel the human’s pulse.
'I've been reliably informed I don't have one,' says Sherlock as regads to his heart.
'We both know that's not quite true,' answers Moriarty correctly. Not just anatomically. By directing the laser sniper at Sherlock's heart and neck, he shows him: 'See. Your pulse is elevated now.' And it isn't elevated because of the pressure caused by Moriarty - Sherlock was perfectly still the whole time. It is elevated because for the first time in his life Sherlock did not feel alone. For the first time in his life he was able to look at someone else's eyes: to see understanding and trust, loyalty and even love - all those things lacking in his life till this point.
This is the very moment when Sherlock realises he can’t live without John. This is the very moment when John realises his life depends entirely on Sherlock. This is the very moment when Moriarty gives Sherlock the hint of his future death - yes, he will ‘burn the heart out of him’. But if he burns just his anatomical heart, it won’t hurt at all - Sherlock didn’t need one till now. He will burn his emotions, his devotion to John, his feelings. He will crash down the only thing Sherlock craves for now - love and understanding. And then he will burn his heart literally, which by then would be quiet painful, for Sherlock wouldn’t like leaving the only thing bringing him now to life.
So, if in ‘A Study in Pink’ and ‘The Blind Banker’ Sherlock showed his brilliant mind, from this moment on he starts showing his emotions. All of them we saw in Series 2. That’s why Sherlock chose the roof-top. Moriarty shoot himself in the head - the master-mind of a consulting criminal he was. Sherlock chose to destroy his whole body cause it is one now - his mind and his heart. Once separate, now they are one. Because of John.