I loved this quote the moment I heard it. 30 Best Sherlock Quotes from 'The Six Thatchers' - Sherlock Holmes: Intuitions are not to be ignored, John. They represent data processed too fast for the conscious mind to comprehend.
I bet he said this because he knew how bad he hurt john when he died. And in his eyes, john is the strongest person he ever met. So if it can hurt john that badly, he couldn't imagine the pain someone "normal" would experience.