This gif kills me every time I see it. I mean, when you think about it, Sherlock isn’t worried about his vision and the hound at all. All of his life he’s been called a freak and told to piss off and people have hated him because he can tell them their life story with a single glance. And then John comes along and he’s the only one who thinks Holmes is amazing and brilliant, and then Holmes sees the hound and he doesn’t want to tell John and then he does and he’s terrified that John will think there’s something wrong with him. He’s not upset because he can’t trust his eyes. He’s Sherlock Holmes, he knows that he just has to look at it logically and it will all make sense. But he doesn’t know that John won’t decide that he’s a nutter and leave him. I think that’s half the reason he decides to test the drug hypothesis on John. He wants John to see it too so that John won’t think Sherlock is crazy.
I completely disagree. Sherlock has spent the entirety of his life valuing his intellectual advantages above all else. His work is reliant on his ability to use a combination of logical theory and physical fact to reach the conclusions he seeks, thus enabling him to solve crime efficiently. Being able to differentiate between the possible and the impossible is absolutely vital to his process. Then, all of a sudden, his belief system is turned upside down as he is forced to address the possibility that either: A) impossibly huge demonic dogs exist, or B) he’s going crazy; the mind he prides himself on is flawed, and is subject to instability. His frustration in this scene has very little -if anything at all- to do with John doubting him. He doesn’t feel comfortable with anyone, himself included, doubting his ability to perform. He prides himself on his intellect. Reason and logic are incredibly important to him, but in this episode he encounters the impossible. He can no longer rely on his own mind, and that is what frightens him above all.