John Watson swears a lot. There's no denying that. What fascinates Sherlock though, is how John swears. Some people are incredibly banal and unimaginative when it comes to their cursing, but thankfully John is not one of them. Full of surprises, that man. Sherlock himself rarely resorts to blue language of any sort, being perfectly capable of expressing his dismay in a wider array of socially-acceptable terms, but he's learned to appreciate how John swears.
The average person will rely on a few choice words, interspersing them with their regular dialogue. Fuck, bloody, shite, etc. A person raised in a particularly Christian household will often have the benefit of words stemming from their religious views to sprinkle in - Hell, damn.
John, however, has turned the vulgar and unpleasant habit of cursing into an art form. What could have been a simple exclamation of "Cock!" in his capable hands becomes the much wordier "Bloody buggering cock."
Over the years, Sherlock has made a habit of studying John and his amusing verbal reactions to things that upset him. Generally, he starts out calm, using words appropriate for public consumption. The angrier he gets though, the worse he gets. Sherlock can always tell when he's really angry, that's when he resorts to a patented combination of bodily functions, random nouns, and pure, unadulterated blasphemy.