John asked Sherlock to be his best man so Sherlock did everything he could to make his wedding perfect, despite the fact that he finds it extremely hard (into battle), he tried to write the best speech ever (unforgettable) which revolved entirely around John, only John, always John, the only feature of interest, the depth and complexity of his jumpers, he has a great singing voice, the warmth and constancy of his friendship, he’s saved his life so many times and in so many ways… and he composed a song for the waltz and he taught John how to dance and he watched John dancing with Mary as he played the violin for him. And then he made a few jokes and looked at John laughing until he couldn’t do it any longer. 

And no one was going to dance with him so there was no point in staying. He had done everything he could to make John’s wedding perfect. So he left. (who leaves a wedding early?) And one month later, he was back on drugs.