The reason some chords or progressions sound "sad" or "happy" for some people is mostly cultural and based on association. And you can rewire your brain to get

The reason some chords or progressions sound "sad" or "happy" for some people is mostly cultural and based on association. And you can rewire your brain to get rid of those associations. It's like saying chocolate taste is happy and lead taste is sad, and complaining chemistry doesn't have an answer as to why. I've never met a jazz pianist that for example will work under the assumption that a 2-5-1 sounds sad-transition-happy, it sounds like a 2-5-1 because the pianist has a sophisticated enough musical understanding to have a concept on their brain that is a 2-5-1 and has a specific sound that is associated to it. It's the whole deal with ear training intervals, scales and chords. The whole major happy minor sad thing is like a pedagogy trick to get people started into getting those kind of sounds into their language. Same thing with intervals, a minor third is maybe sad but then a whole or a half step on the major scale are what? super sad? they sound like a train wreck harmonically but happy when you play them all together on the scale? What's happening is that people just associate musical phrases, sounds, harmony, to stuff and that's kinda outside of musical theoretic research and more like getting into the fields of cognitive science or psychology. Have you actually asked any music theorist about this? I would be surprised if you hadn't gotten this kind of answer because in my experience talking to working theorists over lunch or on social events they usually agree on this.