Why is Watson "always oppressed with a sense of [his] own stupidity in [his] dealings with Sherlock Holmes

I trust that I am not more dense than my neighbor, but I was always oppressed with a sense of my own stupidity in my dealings with Sherlock Holmes. Here I had heard what he had heard, I had seen what he had seen, and yet from his words it was evident that he saw clearly not only what had happened, but what was about to happen, while to me the whole business was still confused and grotesque." (Lengue. 159)