This JUST occurred to me while writing a reply on my "why no magic to fix joyce" post and okay --
( before i go any further i want to say i don't like the "joyce got a tumor because of dawn" theory at all. if that was a thing buffy would have found a magical trace on her during the trance and everyone else would be just as likely to have issues since they were all magically modified. none of what i'm about to talk about should link into that theory at all bc it's dumb. the reason it's so dumb to me is one 1. it reeks of dawn haterism 2. it's copium. )
The supernatural aspects of Buffy tend to be metaphors or thematically tied to her human struggles. It started as "high school as horror movie" and moved into "young adult life via the lens of urban fantasy". In season 5 buffy's two biggest struggles are Glory and her mother's brain tumor.
Sometimes people get sick. That's life. That's the world. That's how it is. Sometimes bad things happen naturally, just as you live your life and there's no big mystical trigger and no bibiddy bobbity cure. And Joss is very much an absurdist and a nihilist, just look at convos like in Amends or Eternity (i think?) or Untouched. Where there's talk about how nothing matters, we're not special the world is big and cruel and impossible, but that pointlessness is what gives us the capacity to make meaning etc. If you want to get more on this what the Passion of the Nerd's Buffy and Angel guides, he gets into Joss' whole philosophy end of things. We see her having to grapple with how to handle real life issues as Buffy-Buffy and not Slayer-Buffy a few other times, like when she thought she'd killed Ted.
But that's why Joyce gets sick, because the world is cruel and uncaring and there's nothing Buffy can do to save her mom from an uncaring god--
AND THAT'S WHAT I JUST REALIZED.
Buffy is struggling with the very real human powerlessness of having a loved one be like, dangerously sick. Buffy cannot defeat Glory because Glory, as a god, is simply too high above her pay grade so she is completely helpless as the Slayer.
If the Master in season 1 is adulthood, and Angelus is the evil boyfriend, and the mayor is high school/paternalism and Adam/the Initiative is conformity/the government then Glory is thematically god. This is also why I felt to the need to say I don't connect this to the "Dawn gave Joyce a tumor" theory because someone is gonna be like "oh but if Glory thematically represents god then that means Dawn is-- blah blah blah" and no. Multiple things can be going on at once and themes/metaphors do not need to complete a full checklist of possible uses. Dawn, Joyce/Buffy, and Glory can also be seen via the lens of adopted verse biological family which ties into Family, "it's Summers blood", the "why didn't you tell me what I really was?" freak out. But Dawn and Glory are also not bio fam so like you know, metaphors aren't steel here.
But it just makes a lot of sense in that in the season where Buffy is confronting the scariest kind of normal human shit imaginable she's up against a god. In situations like that of the Summers' family, people will curse god or pray because there's just nothing else they can do. It's a deep chasm of helplessness that no one can fight against. Your loved one will make it or they won't. And even if they live there could be major issues. It's all beyond the scope of what man can control. And it's reflected in her supernatural struggle with Glorificus, a hellgod who EATS BRAINS, has the same rough hair color as her mom, who wants to kill her sister and Buffy is too powerless to stop her.
