I think that people like to think that they’re smart,” he says. “And unambiguously happy-sounding music has become, over time, to sound more like a cliche. If you think of children’s music like ‘Twinkle Twinkle Little Star’ or ‘The Wheels on the Bus,’ those are all fast and major, and so there’s a sense in which unambiguously happy-sounding songs sound childish to contemporary ears. I think there’s a sense in which something that sounds purely happy, in particular, has a connotation of naivete.
Sounds like this fascinating shift began sometime in the 70s, around the time I was born.