A disordered mind
Wow, what a stream of revelations we have here. Bob is right to say that OCD is something quite different from "obsessiveness", and I get very angry when people talk about being "a bit obsessive-compulsive" and so on, when Goddammit! They haven't the faintest idea what it means. Sometimes I wish I could just turn my whole brain off and put it away in storage for a long, long, time. Someone asked earlier if OCD is related to autistic disorders, and there is evidence that it is. There is a greater overlap of occurrences of OCD and Asperger's than you would statistically expect, and indeed I have had my own brain photographed, rather excitingly, in an experiment to test this. Asperger's is the syndrome suffered by the narrator in The Curious Incident of the Dog in the Night-Time and one of the reasons I was so moved by that book was that, God help me, I identified with practically every aspect of him, except for being unable to read expressions, which is a defining characteristic of Aspergers. Many of the things that character does, such as counting, refusing to touch certain things, etc. are traits associated with OCD. Personally I think OCD, autism, and Asperger's are all somehow related, but people like me are very much on the "high-functioning" end of that spectrum, which means we can pass ourselves off as normal human beings. Well, most of the time.
By the way, it's true that some of these disorders are "over-diagnosed", but they are "under-diagnosed" as well. One in twenty adults in the UK has ADD, but the vast majority are never diagnosed.