Once again… I can not with this scene.
One of the more deliberate moments where the watcher witnesses the depth of Sherlock’s pining. Sherlock is referring to John’d romanticism in relation to his blog.. which, let’s be clear, is a blog revolving around Sherlock. John isn’t romanticizing crime, or murder, or the generality of cases… he’s romanticizing Sherlock, his mind, his acuity, their time together, who Sherlock is specifically as a man and John responds to Sherlock’s identity by writing about it with great depth and affection.
Up until now Sherlock has been the primary recipient of John’s romantic attentions. Not outright physical romance, but something with perhaps even far greater intimacy. John accounting Sherlock with beautiful sincerity, and genuine respect, his words making Sherlock into something desirable. (Which in turn, Sherlock returns that same level of romanticism in his best man’s speech.)
Sherlock was probably content just having that from John. Having love expressed through the safe medium of John’s blog. Sherlock’s small hesitation and and averting his eyes so he’s not looking right at John, pursing his lips. His tone and expression is a cocktail mix of regret, guilt over having these feelings in the middle of this event, and internalized longing.