Why magic at 80 db?

Been hanging out with a professional sound engineer off and on, tweaking
my speaker placement, talking trash and such -- and it turns out that the
golden number where virtually all music is mastered in the studio is at
83db. He can pick it out be ear, just magically knows when it is at the right
level, as that's where it hits the grove and does what it was recorded and
mastered to do. (And must admit, the amount of information he can hear
and explain about the mastering and mixing decisions behind any given
track is astounding). But if it sounds like it all comes together and the
magic happens just north of 80db, that's because it was mixed that way
very much on purpose. Short version: all our collective fascination with
hardware permutations notwithstanding, i suspect this one is pretty much a
software issue. (Personally, I find 83db to be rather loud in my room,
louder than I listen 90% of the time. Feel that I get all I need from lower
volumes -- but, must admit, once you spend some time at or around that
baseline level where things were mastered, it starts to get awfully
addictive....).

Looking back at my post above, occurs to me that I may have stumbled into an old chicken and egg conundrum: do things sound best at just above 80db because they were mastered at that level to sound their best, or are things mastered at 80db because that's just inherently where things sound their best to the human ear over a prolonged period? Dunno, but I suspect ultimately a mix of both. But, either way, seems like a known factor.

Funny, circular definition issue, I've been told that one of the easiest ways to dial in ~83db "freehand" is that it will be just a touch louder than anyone could or would comfortable want to talk over.... Seems like we've all triangulated the same, relatively narrow volume range coming at it from entirely different directions.

To follow up, here's some extended discussion from a mixing/mastering forum concerning how to set an 83db "reference level", how fatigue plays out at that level, different opinions regarding the potential for hearing damage based on time of exposure, etc. You guessed it, there's a forum for everything. If you're interested, you can also find extended discussions of mastering styles and theories, the benefits and evils of "compression glue", and all manner of other stuff that goes into the decision tree behind any given track. Personally, spent a bit of time digging into this stuff recently, but ultimately decided that ignorance is bliss on this one. (Figure I enjoy sausages a whole lot more for not knowing how they're made, either. But that's just me.)