"Alone," Sherlock announces, "protects me. Alone keeps me safe; I am fine with alone. It's not being alone that got me into this mess in the first place, knew it was a bad idea. Probably still is."

John Watson possibly (probably) saved his life that first case, maybe, maybe yes, but that was more serendipitous than expected, wasn't it? He knew. Sherlock knew, knew from his own paltry experience that getting involved was dangerous. That tangling himself up in anything but his own problems would end disastrously.

Maybe that's why he did it. Could be dangerous. Maybe they were both doomed from the start.

"Been trying to get rid of them for ages. People. Family; family is an awful mess, can't be bothered. Forgot people make their own. Suppose it was a miscalculation."