It comes from 'relation"ship"' and has no consistency in who or why might be paired together.
I remember years ago on Tumblr one of the big BBC Sherlock ships was Moriarty and Sebastian Moran, except Moran wasn't even in the show properly, if at all.
The fandom assumed a sniper [which is the sole connection to Moran lore wise] at the end of season 2 was Moran and then universally decided he was played by Michael Fassbender so there were gif mashups of Andrew Scott who played Moriarty and Michael Fassbender from various other films and shows since he wasn't even in the program.
This all happened during the "break" between season 2 and 3 which lasted so long that the shippers became rooted in this headcanon because no actual material came out which countered it.
BBC's own fault for taking fucking years just to make 3 episodes each season.
Season 3 poked fun at some of those headcanons with the Sherlock fan group and their theories on what had happened. A literal tumblr ham planet suggested Sherlock and Moriarty were trolling John together "and then they kissed". At least the rest of the group looked at the fujoshi like the mental case she clearly was but as you point out a lot of the problems with that show stem from how long it took to make so few episodes.
It comes from 'relation"ship"' and has no consistency in who or why might be paired together.
I remember years ago on Tumblr one of the big BBC Sherlock ships was Moriarty and Sebastian Moran, except Moran wasn't even in the show properly, if at all. The fandom assumed a sniper [which is the sole connection to Moran lore wise] at the end of season 2 was Moran and then universally decided he was played by Michael Fassbender so there were gif mashups of Andrew Scott who played Moriarty and Michael Fassbender from various other films and shows since he wasn't even in the program. This all happened during the "break" between season 2 and 3 which lasted so long that the shippers became rooted in this headcanon because no actual material came out which countered it.
BBC's own fault for taking fucking years just to make 3 episodes each season.
Season 3 poked fun at some of those headcanons with the Sherlock fan group and their theories on what had happened. A literal tumblr ham planet suggested Sherlock and Moriarty were trolling John together "and then they kissed". At least the rest of the group looked at the fujoshi like the mental case she clearly was but as you point out a lot of the problems with that show stem from how long it took to make so few episodes.