I am not sure about that. 
- He only leaves the wedding after realising that Janine is dancing with someone else. You can see it clearly in his face. And a moment before when he is standing alone on the dance floor looking a bit lost. 
- As for David: Mary und John made a guest list and Sherlock checked if someone did not like her (like the person who bought the card at the petrol station). I suppose he checked everyone and therefore noticed that David still seemed to be after her. (My question about this would be why she invites her ex who follows her in various social media to her wedding). 
- I think if he were there for work it would destroy the whole idea of the episode basically being a declaration of friendship (or love, for the Johnlockers). The wonderful thing is how the case emerges in  the course of his best man speech and how he tries to do his duty towards John while at the same time solving a case (and allow John to save a life). 
 

And was so much better.

SolarSystem wrote:

SusiGo wrote:

I think if you look closely at his face during the conversation with John and Mary and his move towards Janine before he realises she is dancing with someone else, you can see that there is more. No heartbreak, that would be saying far too much, but remember him telling Janine how much he loved to dance. He prepared for a waltz with her and did not get the opportunity. And he felt that he did not belong. 

I agree. He was looking for Janine and even almost took a step into her direction. And when he realised that she was dancing with this other guy, there is a very particular expression on his face for just a second, as if he were thinking "Ah, okay, fine. So everything back to normal now." And then everything slowly goes back to 'normal' again.
But the way in which he is shown in the middle of that dancing crowd is very telling, in my opinion. I can practically feel the loneliness, and even if it doesn't come as a real surprise to Sherlock... he still looks lonely there for a moment.
 

 
Couldnt agree more..

I actually think that in spite of the sadness implicit in Sherlock's leaving, the ending had kind of a happy-ending feel to it, (the wedding has gone off, John's other friend and leader has been saved, and while Sherlock may be sad about John getting married he's choosing to channel it in a positive way and be in touch with his love for John). It almost feels like a series finale.

And so HLV has that WHAM effect.

I didn't catch that the lyrics "I didn't even know her name" were a clue about Mary, of course! The song doesn't fit John and Mary otherwise...would fit Sherlock and Irene if you believe they "had dinner."

 

Sherlock looked lonesome yet at the same time relieved that his role as a best man is finally over (he hates marriages so it was probably a live torture for him). He was determined to fulfill his duty, he managed that perfectly and it was time for him to leave when his work was done.
 

He always looks cool!

What I got from the ending of TSoT was a Sherlock haunted by Mrs. Hudson's words, "The end of an era." He seemed saddned that his partnership with John was possibly over, at least as it had been, and then he seemed like he steeled himself to go back to life alone. Sort of remiscent of the scene where he's about to get dressed for the wedding, and is like, "into battle", as if he's putting on armor. He knows it's going to hurt. But he's doing it anyway, for John.

I read through the copious pages of comments on this thread and one thing stuck out at me, that they're all written from 'John's' perspective, that is of someone who is more 'normal' on the cognitive-emotional spectrum.  I am on the high functioning asperger side of the spectrum, which is also where I believe Sherlock sits (I definitely don't buy the sociopath thing). He's way cleverer than I am, of course, but what attracted me to the show was discovering a character who thinks like I do.  There have been so many little and not so little scenes in the three series where it was like seeing my thought patterns and how people react to me splashed out on the screen.

So that preface done, I have been in Sherlock's exact shoes in this episode, minus the murder and mayhem and drunk stag night, and I was the maid of honour, not the best man. Coincidence of all coincidences, the bridesmaids at my best friend's wedding had lilac dresses identical to those worn in this episode, two with straps, one strapless. Weird!

Now, onto dejected Sherlock.

Imagine being really clever but with no social skills whatsoever so you have been the target of bullies your whole life. You know you're not quite like the others, you don't see the world the same way, don't have the social finesse of your peers. You see the world intellectually, not emotionally, so close relationships are difficult.

And then one day you meet an incredibly special person you just click with. It's not romantic. Romance means nothing to you, is beyond your understanding, and is not something you seek. It is much more important that this person accepts you regardless of all your faults and perhaps even for those faults.

Crazy adventures ensue. You bleed together, laugh together, even cry. You see each other at your best and at your worse. You are not you alone anymore, but You and That Other Person. Your names run together in the same breath. You are Best Friends. For someone who never had a real friend, to have a Best Friend shatters your view of the world.

But unlike you, your best friend is 'normal' and wants those things 'normal' people want. They are not satisfied by intellectual pursuits, but crave that romantic love that bewilders you.

And that is how, after a number of years, you find yourself at your Best Friend's wedding, smiling on the outside. You do all the duties expected of the Best Friend, like a prefectly rehearsed scene from a play.

But you have never felt so alone in your life, because you understand now what it means to be lonely. You know that it's all over, that a chapter has closed, and you are once again alone in a world that doesn't want to understand you. But perhaps most important of all, it might be a world you don't understand either, but your Best Friend taught you it is a world worth being a part of.

At the end of TSOT, Sherlock is confronted with the fact that life as he knew it ended on the roof of Bart's hospital two years prior and that he was right back where he was before the taxi driver case. More mature, certainly, but alone, and keenly aware of it. Losing John to Mary was one thing, but now that there's a baby on the way, Sherlock knows there is absolutely no hope of continuing a semblance of their old relationship. John has well and truly moved on.

So John and Mary are off dancing. Molly is with Tom. Janine's met up with the geek. What is Sherlock going to do? Dance with Mrs. Hudson? Stand on the sidelines until someone notices how pathetic he is? He's done his job, and done it very well. Time to go.

Mary