Sherlock always wondered what a physical touch meant. Sure, he had observed women with their mates, how she would reach out to him just to feel him, just to know he was there.
Sherlock had always thought that the act of physically being with someone was pointless - something only a weak person would need.
Sherlock had never thought of himself as weak. His heart was almost stone. Of course, that's not true. The heart is an organ made of muscle and tissue. It provided life to the body. It certainly didn't make Sherlock weak.
Sherlock had never thought Dr. John. H. Watson would make him weak. Ever since Moriarty's first assault on John, by the pool, Sherlock had needed it. Needed that touch. Sherlock was weak. His heart of muscle and tissue and power was failing him. Sherlock longed only for the touch on one person.
This longing would keep him up all night, driving him to exhaustion. This longing would keep him from eating, making him starve for answers. Sherlock didn't want this, but it was uncontrollable. Every day he battled with the urge to feel.
I suppose it's no secret. Our dear consulting detective was made weak by our dear army doctor.
When it came to John, weak was what Sherlock wanted to be.