hudders-and-hiddles:

the drugs thing i never believed for a moment. anyway, you wouldn’t care if it was exposed, would you? but look how you care about john watson…

the clear implication is that magnussen has to choose to utilize a pressure point that sherlock wouldn’t want revealed. so if he chooses john, there must be some aspect of their relationship, of sherlock’s feelings about him, that sherlock doesn’t want the world to know. everyone already knows they’re friends, so it would be worthless to expose that. so then the question becomes what else could sherlock possibly want to keep hidden?

in short, if sherlock’s care for john was entirely platonic, magnussen wouldn’t use john as his pressure point.