I think people are just afraid of different things. Anyone who is afraid of the judgment of others, to the point that it interferes with their life, is going to be diagnosed with SA. They're going to have a lot of similar behaviors because they're scared of the same kind of stimulation. Arachnophobes are all going to have similar behaviors, too, and those behaviors are going to differ from the behaviors of a person who's afraid of heights. The behaviors of a social phobe and an arachnophobe differ because the objects they fear differ. You can call these different disorders if you want, but I'm not sure there's much difference between them beyond the way the object impacts their behavior. GA is probably just a more generalized kind of fear of "what could happen".
If I were to be diagnosed, I'd probably be diagnosed with SAD, despite the fact that I don't really feel like that's my problem. I'm afraid of physical violence, and I'm afraid of the OCD that it triggers; it keeps me away from people; but they'll probably diagnose my behavior, not the reason for it. I could probably have a diagnosis for complex PTSD, since I have all the symptoms, but I don't have any memory of a specific trauma, so I wouldn't get the diagnosis. Even though I think it fits better than SAD. They might give me GAD instead, even though I don't think it really fits, since I never worry about all kinds of things I could be worrying about.
Another example: I meet the criteria for BDD, but the diagnosis hinges on the incredibly subjective decision whether or not I'm actually ugly enough for my feelings to be justified. Maybe it's just my gender dysphoria; but then, half the trans community seems convinced I'm not even trans. So what are you going to do? There isn't a single diagnosis that fits me perfectly; I seem to be missing some important element from all of them. But I'm a complete ****ing disaster. Depending on the therapist, I could get diagnosed with half a dozen disorders or none at all. If it's that subjective, what are they actually diagnosing?
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