Why does there seem to be something deeply true about Wagner's Siegfried? Of course, it has its deep roots, particular to its story arc; but because it was made, almost if in plain sight of everyone, it rings even truer.
To an useful first-order approximation, there are two competing, but complementary Proposal Theories. PT 1 is that great art can and does reach down (or upwards) to something truly essential and essentially true: something comparable to the genetic of Being or civilization or meaning. PT 2 is that we are accultured into higher-order syntaxes of myths and chords and notes and intervals that enable the composer to express the meaning he intends to express with bare symbols; and onto a more sociological note, that we know opera to be a "respectable" form, so we are predisposed to recognize some depth in it.
A reasonable position will typically combine or vacillate between PT 1 and PT 2. As much as we try to keep a straight philosophical mind that protects us from vacuous platonism, it is true that not all high-order, high-complexity hierarchical systems of symbols ring as true to us as great music.
My proposal second-order approximation is to embed one position into the other. From the formalist position where truth in art emanates from the system of syntaxes in which it is written, we take the notion of hierarchies -- strata, to use fashionable terminology. We know that pitch is sped-up rhythm and chords and intervals are made of pitches; we know that the larger orchestral horns sound dark because they are lower-pitched but also because we have heard much orchestral music that uses them so. We then pile reasons upon reasons, layers of meaning-making. But what makes these layers stick together and sediment, while others barely equilibrate for a few minutes?
I claim: what "glues" these layers is sedimentation and selection -- not construction or reference to an outside essential realm. Which is to say: the apparent essential truth is a structural feature of hierarchies. But what makes each layer a stable consolidated layer? Further "interstrata" of truth -- to an extent that we could very well say that layers of syntax are the glue that fixes layers of truth in place. Folds upon folds upon folds of syntax and meaning supporting each other: isn't this what we actually hear in music? Each note an entire civilizational project, each chord a concept of what is true about the ongoing harmonic narrative?