We tend to think of intervallic consonance on a relative scale – maybe octaves at one end, minor ninths on the other. In between you have every other interval – M3, P5, TT.
It has long been my sense, though, that relative consonance is built on a sliding scale. Where one person draws the line between consonance and dissonance is not necessarily the same for their neighbor.
It depends, essentially, on adaptation. The music we listen to informs our sense of consonance and dissonance, and we use our experience to draw conclusions about new music. But this line is not static. We can inoculate ourselves to appreciate more dissonance by interacting with it – hearing it, playing it, singing it. Over time, the “devil in music” becomes just another tritone – easy to tune, once you’ve got the ear for it.
Want to sing sophisticated harmonic music? It’s not enough to learn your part. You’ve got to learn to love the dissonance. Without loving it, your ear will gravitate away from it rather than toward it.
I don’t think we’ll ever get to 12-tone nursery song, as Stravinsky allegedly thought, but I do think that by gradually and consistently expanding the tonal palette we experience, we can learn to love new sounds, and teach our students to do the same.