There’s evidence (from 2008, see Holman et al.) that some people have quite a bit too much miscellaneous body pain (a.k.a. fibromyalgia) due to intermittent compression of the spinal cord. The compression fires up the sympathetic nervous system, but has no other immediate/obvious consequences. This is clinically subtle and tricky (and expensive) to diagnose unless you know exactly what to look for.
(Note that this is not a posture problem. You’re not going to get spinal cord compression in a healthy neck. No postural habit would plausibly do that. But there are a variety of pathologies and injuries that can result in spinal cord compression. Like arthritis, most straightforwardly.)
More study needed, of course, and I haven’t yet checked to see if there’s newer research on this topic — this is just a quick look at something that really grabbed my attention. Because, if true, this would explain a lot. And illuminating sneaky mechanisms of pain is exactly what we hope will characterise the next few decades of pain science: that we will finally start reaching beyond the low-hanging fruit, and look where the light is not good, and start to figure out why pain treatment has generally been such a difficult medical challenge.
I’ll be following up on this. Hat tip to Brian James for pointing this one out to me.