What worries me was that line from Sherlock about not quite being sure why Watson made him better, and how he might solve that mystery in time -- followed by an uneasy look between them. That felt uncomfortably like an Unresolved Sexual Tension beat. I really hope they don't go there.
I did not read that line that way at all. Rather, I was reminded of this passage from The Hound of the Baskervilles
"I am bound to say that in all the accounts which you have been so good as to give of my own small achievements you have habitually underrated your own abilities. It may be that you are not yourself luminous, but you are a conductor of light. Some people without possessing genius have a remarkable power of stimulating it. I confess, my dear fellow, that I am very much in your debt."
I think Holmes, at this point in his relationship with Watson, is beginning to recognize that she is his conductor of light but it's still so early that he can't put it into words.
That's how I read the scene, anyway.