But in the fanfic, I'm reading, Sherlock is nearly killed and gets a blood transfusion from Watson (they're the same type, and there was a major highway accident just before Sherlock arrived at the hospital). And this makes him ridiculously happy because he's always fantasized about trading blood with Watson so a bit of each of them will be running under the other person's skin. And so he wants Watson to take a vial of his blood as a kind of gift.
This is Sherlock being romantic. And Watson thinks it's certifiable, but also promises to consider it if Sherlock still wants to do it when he's no longer high on opiates. Which is beyond sweet in its own way. Because the idea of blood transfusions as romantic gifts is... odd. Disgusting in its own way.
And also: without a doubt the sweetest thing I've ever seen. It's so Sherlock. and it's also the kind of thing I can see meaning a lot to me, if it ever happened to me.
Yes, I am an odd one. I'm also grinning from ear to ear, and it's all I can do to keep from squealing out loud. Because really, it's the sweetest, most endearing thing I've ever read.