Or perhaps this way: John finds Sherlock and tells him to go back to hospital. Sherlock explains the situation and wants to sort things out with Mary first. He knows it will take them some time to get back together, but he wants John to make a decision. Sherlock will have to go back to hospital afterwards for a long time. That will give John and Mary the time they need. Once Sherlock's health is restored, he can tackle the Magnussen case immediately, and he will hopefully have his old John back, or at least they'll be on a better basis than in the beginning of HLV. The knowledge that his friend is very ill and will be taken back to hospital soon puts additional pressure on John, which is why he's extremely upset and a bit helpless trying to do the right thing.

Apart from Mrs Hudson, there are three people in the room in that scene. Mary, who obviously thinks what she's done was absolutely right and the only possible way to act. Sherlock, who thinks what he and Mary did was absolutely right. And John, who probably thinks they've both gone insane.
That mutual understanding between Sherlock and Mary seems very strange to me if "she phoned the ambulance" is really the only explanation, that's why I came up with the "Mary saving Sherlock" theory in the other thread. There seems to be something John doesn't know, something they don't tell him, and perhaps he senses it. He has a long enough experience in dealing with exceptionally clever people who keep secrets from him. And he has always chosen to trust them more or less blindly because he sees the good in them that other people don't see. He is the one who calls Sherlock a hero again and again. After all, the reason why he "chose" Mary was perhaps not that she's dangerous, but that she's a good person, and he can see that even when nobody else can see it anymore?

Personally, I didn't understand the "you won't need morphine" line as a serious threat, more like a retaliation because of Sherlock's drug problem. If I'm not mistaken, Sherlock is taking far too much of that stuff. In the scene where they confront Mary, there are a glass bottle and a plastic box to see which probably contained pills. The bottle is already empty, and when they arrive at Baker Street Sherlock says he's run out.

When the ambulance arrives Sherlock says he's pretty sure he's bleeding internally, but he's not a doctor, so it might not be that dramatic after all.

If (!) Sherlock was in danger of killing himself and Mary was the only one who noticed it and knew what to do about it, then she would really be a great character. But we don't know.
I feel like rewatching all the episodes looking for "evidence" for and against this, but I don't have the time at the moment. Maybe tomorrow.
So far it's just a thought. Could be total nonsense.

I imagine it works this way: Magnussen has an interest in keeping both Mary and Sherlock alive, because that's how his system of leverage works - Mary>John>Sherlock>Mycroft. Mary doesn't know that. She's only interested in keeping herself and her family safe, and that includes Sherlock. But he has developed a tendency to behaving like an idiot lately and is about to ruin everything. Killing Sherlock is totally out of the question for her. Just incapacitating him wouldn't solve the general problem. So she decides to teach him a lesson.
Sherlock is at first shocked and disappointed that Mary seemingly wanted to kill him. His survival instincts, or the voice of the "good" side of him, tell him to try to stay alive. The "evil" side (in the form of Moriarty) says, "why don't you just die". He's more on Moriarty's side, but in the very last second he suddenly remembers John, realises that Mary doesn't want Sherlock to die at all, and deduces how Magnussen used all three of them to get to Mycroft. So he wakes up and gets back into the game. He repairs the John-Mary-Sherlock-relationship and then tries to turn the system of leverage around, using Mycroft to protect the other half of the chain.

Swanpride wrote:

I guess the difference between Mary and Sherlock is that Mary is trained to pay attention to details and keep them in mind in case she needs them later on - especially stuff like schedules and architectural outlays. Sherlock on the other hand trained himself to notice details, disregard what is not important and come to conclusions based on said details. In the disregarding part lays the danger, because he can't know what will be important at one point. But I bet he notices more than Mary and is able to reach conclusions which would never occur to her.

I guess John is the one with the emotional intelligence...even if it tends to trip him up in certain situations.

 
Yes, you have a point there.
On the other hand ... Mary managed to distract Sherlock from her connection to Magnussen and the problems with her past for a long time, using the feelings he has for John and her and his love of solving crimes. Empathy is necessary for that. And in contrast to Sherlock, Mary says she has "lots of friends", so she's able to easily win people's hearts.

@Willow: John forgave Mary after Sherlock demonstrated to him that he trusted Mary completely, so much that he put John in a position where she could have shot him, thinking he was Sherlock.
And the thought of "breaking" John wouldn't even occur to Sherlock. That's Mary being considerate, Sherlock being inconsiderate.

MahnSherlolly03 wrote:

This is a really lovely idea and I may have to accept it as headcanon.

 
That's nice of you to say. I know my theory could be total nonsense based on emotional overreacting, but I just think it's nicer than always seeing everything so negatively.

I was hesitating to post this because it is a rather daring theory, and I'll probably have my head bitten off. But the more I think about it, and considering things I pointed out here: http://sherlock.boardhost.com/viewtopic.php?id=4631 , the more likely it seems to me that it's true.
I know everyone's complaining about Mary because she put Sherlock's life in danger, but after thinking about it for a while I've come to a different conclusion.
I think Sherlock is a bit of a dreamer. He sees his life as a fairy tale with dragons and princesses, or as a play. Like he refers to the Reichenbach fall: "Like a scene from a play." I bet he thought that was great - Sherlock in the role of the dying hero. There was even a tiny little risk that he could actually have died (if he hadn't hit the airbag, or landed in a wrong way). Tragic. He would have died while doing his work, like a soldier in his uniform (like Major Sholto), and it would have been his own fault, instead of his enemies winning over him.

He was about to do something similar in PINK before John shot the cabbie. Sherlock was pretty sure he had chosen the right pill, but there was a risk left that it could be deadly poison.

So in HLV, when he is sure that he has no place in John's and Mary's life anymore, Sherlock is full of self-pity and totally pulls off the "heading towards a tragic death" thing. He's using drugs again and takes dangerous cases with an increasing risk that he'll get killed sooner or later.

In the CAM building, when he sees the situation with Magnussen and the assassin, Sherlock must have immediately concluded that the last thing the assassin wants is an eye-witness. So it's highly dangerous to approach her. (Even when he still thinks it's Lady Smallwood.) But he takes the risk and puts himself in danger, with a high probability that he'll get killed. After realising that it's Mary, he continues doing that - approaching her although she's seriously threatening him. Finally a chance f

Also, at the wedding Mary is wearing a white dress (of course), while in the scenes where Sherlock sees her as a "liar", and when she shoots him, she's dressed in black.

I don't know if this has been pointed out before, but today because of "Swan or Sydney Opera House" and dancing and ballet I thought of Swan Lake. From the German Wikipedia article I got that the story of the ballet (music composed by Tchaikovsky btw) is about a prince named Siegfried who has to choose a bride. He wants to marry the Swan Queen Odette, who has been turned into a swan by the evil sorcerer Rotbart (which means "Redbeard"). Only true love can break the spell. At a festive ball the prince expects to see Odette again, but instead the sorcerer appears with a dark woman (Odile) who looks just like Odette. The prince falls for the trick and swears he loves Odile, and dances with her. Then he realises what he's done and runs back to Odette. In some adaptations they both die, in others one of them dies, or they both live happily ever after.
So, do you think this could be an intentional reference? Mary would be Odile/Odette (black and white swan, evil and good), Magnussen would be the "evil sorcerer" who turned her into his creature. If the plot was roughly based on a ballet, that would also explain why S3 is so overly dramatic. In ballet and opera people are never just unhappy or so, they always "die", especially when they lose their true love. And it's Sherlock's perspective. That would be like him, to see life as a play/ballet/opera. (He definitely does a "dying swan" a couple of times.)
I would conclude that Sherlock as a child was a nerdy little boy who liked ballet dancing (I bet his peers hated him ). So much that he named his dog after the evil sorcerer from Swan Lake. When Mycroft asks him if he remembers Redbeard, perhaps that's a hint that Mycroft knows about Magnussen and Mary and that Mary is not what she seems to be. But also not really evil (and Magnussen is not a murderer, and also sometimes useful to Mycroft), otherwise Mycroft would have intervened. He just warns Sherlock not to get involved.
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