In season 4 episode 3, after talking to molly, why does Sherlock break the coffin and act like agonized? I didn't understand why exactly he broke at that point of time.
I found an obscure Tumblr blog which has a video of this under discussion. Unfortunately, it's not embedable here so I'll try to summarise the cast's thoughts and comments from Mark Gatiss. It's only a couple of minutes long.
"I think I'm out of control over Molly Hooper. Now does that mean I love Molly Hooper? I don't think I've made that decision yet".
"I don't think we've ever seen his emotions as out of control as in that moment..but I think that's because he's brought to boiling point by the situation rather than it being some kind of cathartic turning point."
"The smashing of the coffin is interesting because [snip]..I wanted Sherlock to have a kind of release of that kind... of the fury he's feeling of being manipulated like this.
"You know, Euros knows what she's doing, she knows what buttons to push to make Sherlock emotional."
I think Sherlock broke the coffin because it was too much to hurt Molly. His sister Eurus has riched the limit to put Sherlock in front of Molly, when obviously he does not love her, and make her to tell him I love you.