There were times when Sherlock became completely dead to the world. Some people might describe it as 'an outer body experience'. Sherlock described these people as arrogant and stupid. He had a logical mind, his way of thinking lead him to categorize the world around him, he had no need for other people to interfere. He supposed this is why he resented Mycroft. This is however beside the point.
When Sherlock was in one of these 'moods' it was like the world was in complete darkness and the only source of light were pieces of information relevant to the case he was currently working on. He found solace in joining up the little dots of light. Creating the bigger picture, ultimately solving the case. The thrill it gave him, it was his vice. The thing he needed to get him through the day. He needed to find the next dot to join, he needed the satisfaction. Like any vice though, it could become an unhealthy addiction.
This brings us back to the case in point, he didn't see the world around him, didn't realise his basic needs as a human being. Eating, sleeping, all irrelevances that became lost in the darkness. It was all a downward spiral, his own personal black hole.
Then John appeared in his life.
Sherlock could be lost in the darkness, and a hand would touch his shoulder, lips would brush his forehead. A warmth would spread through him melting away the dangerous blackness, bringing him back to reality.
He no longer hunted the next connection with animalistic obsession. He still felt great passion for his cases they still gave him joy but he no longer needed them like a drug. John had broken through that relentless cycle.
John gave him a point of focus something permanent to live for. He no longer survived for the next piece of a puzzle. He existed alongside the cuddly jumpers, warm smiles and comforting cups of tea, with John.