When I hear a major sixth interval (also called a “diminished seventh”) – for example. middle C and the A above it played simultaneously, with no other context – my brain “fills in” a note to “create” a major triad whose root (in this case, F) had been “missing.”
But it should be just as likely for my brain to “fill in” a note to “create” a minor triad, right? (In this case, A minor, by “inserting” an E).
Why do I “hear” the former spontaneously, yet have to force my brain to hear the latter? Is it just because my post-Medieval Western ears have been conditioned to look for major chords as a first option?
Or is it that it’s easier to “fill in” a 1 than a 5? (Maybe because what the brain is really looking for is a third interval – either major of minor – and we tend to hear the top note of an interval as the “melody,” so my brain is saying, "I hear an A…that’s the top of what third? How about F major? Perfect – you’ve already got a C down there, to complete the triad!)
Or is my particular brain just weird?