Sylvester

everyone talks about how sherlock isn’t afraid because he saw the hound in the hollow, and is actually afraid of his feeling, specifically for john.

But there’s something that happens right before they even get to the hollow that backs this up so well!

when sherlock and Henry are walking to Dewers Hollow, Sherlock’s questioning why Henry’s dad was friends with Dr. frankland if their beliefs would have clashed, and henry just goes: “look at you and john.” and then points out that john is straightforward… and sherlock very much isn't.

and sherlock reacts to this.

he gets defensive. confused. almost… unsettled.

This forced him to look at his relationship with john from the outside. To see the imbalance, the difference, the possibility that whatever they are… might not work. Its a sore spot for him.

and then immediately after that, they walk into the hollow.

into the fog. The drugged fog.

so you’ve got sherlock already destabilised, already poked right at the thing he avoids thinking about: how he and john fit together, what they mean to each other, whether he’s too much or too strange for someone like john, and then gets hit with a drug that amplifies your fears.

And so, we get the scene: "Look at me, im afraid John." "I've always been able to keep myself distant. Divorce myself from feelings."

As sherlock has a full panic attack over it.

So it wasn't just the hound that triggered that reaction. But Sherlocks fear that he can’t trust his own mind, that what he feels for john is something uncontrollable and overwhelming and possibly destructive.