After 26-7 years with this ugly monster, I'm coming to a conclusion. (Actually came to it quite some time ago, then acquired a head injury and forgot!)
It is this:

Pain, in and unto itself, causes us to become tired. I don't know the exact science behind it, but dam, if it doesn't take a lot of effort to 'Hurt'!

I've got a fairly well regulated medication regimen.
I should exercise more, but that's more for long term health, imho.
(It surely can't hurt, we definitely need it)>

I also know that depression can put a big weight on us to carry around, and that is exhausting.

So, in my humble little mind, it's those two things, Pain, and the depression that comes with it naturally (if you're in chronic pain, you're depressed), wear us down.
It's the same two things, that draw us to come here, to come together!
We're 'naturally' fighting pain and/or depression.
And, thanks be to God, for those who come here for their loved ones! You all have my respect!

Pete
asb