The absence of Sherlock Holmes in the life of John Watson is similar to the lack of colour in a rainbow. A rainbow is colour. Without colour, a rainbow is not, indeed, a rainbow.
Without Sherlock Holmes, John Watson is not John Watson.
And so when Sherlock Holmes takes a swan dive from a particularly tall building one day and his body brutally kisses the ground and christens it with his blood, John Watson's life is drastically impacted, needless to say.
This is not to say that John is a weak man. Oh no, quite the opposite. John Watson is a loyal man. He is kind and caring, brave to a fault- but he is nothing without his war zone, and Sherlock provided that for him when he needed it most.
If we are to continue with the weather metaphors, then Sherlock Holmes was a fucking tornado, and John's the mess and devastation he left behind.
Quite the impression Sherlock bloody Holmes left on the world, hm? But as John had once heard it said, the marks humans leave are too often scars.
John knows what it's like to have a broken heart, and he has his own physical scars from battles in times past- so he knows what he's saying when he claims that Sherlock's left more of a cavern than a scar; on him, anyway.
That's the frailty of genius: it demands an audience. But what happens when your audience turns against you? The game that Sherlock played would have been much easier if the worst that could happen was for the crowd to boo him off of the stage. Their lives weren't a pantomime though, and John's sure that Moriarty was the only one laughing in the end.
That's the difference between fantasy and reality though. One has a happy ending, and the other has an ending that could be happy or sad or seemingly never ending, or it could come to a stop far too soon and much too abruptly. You never quite know.
John is rain, but Sherlock was his light. You can't have one without the other, because otherwise there's no rainbow.
note: the 'the marks people leave are too often scars' bit is reference to The Fault in our Stars by John Green. (Oh my god you should read it.)
Side note: this is a repost of this story as the original was deleted because i forgot to remove the f word from the summary when i was copying it over from ao3. :( the comment that was left anonymously has been replaced by me, just because it was nice feedback and yeah.