Hmm... I tried to find out more about the "valence" attribute from spotify. I assume both of these values are coming from machine learning algos. Spotify describes valence as how "positive" something feels, but the word valence comes from chemistry - the linking between atoms. It's also used in linguistics to describe the linking of terms (in "charlie gave the big, jolly, man apples", 'gave' is linked to 'charlie' and 'apples' and 'man', 'the', 'big' and 'jolly' are also linked to 'man', but not to 'gave' as they refer to 'man' not the act of giving, so 'gave' is connected to 3 words, valence of 3, man is connected to 4 words, valence of 4).

Since the linguistic definition of valence doesn't at all seem to fit 'positivity', I assume valence here is tonal or rhythmic, but it's not a common descriptor in music generally. It could be rhythmic synchronization, or an inverse of syncopation, but that wouldn't generally make something feel positive, so my best guess is tonal, and it's a machine learning algorithm trying to fit the pitches to the tonic / major scale. Any song in a major key can be rewritten exactly the same in a minor key. A minor and C major use all of the exact same notes, but if the song tends toward C as "home" it's Major and happy sounding, if it tends toward A as "home" it's minor and more sad or whatever. You can't just have an algo look at what notes are used to determine if it's major or minor, you would more likely look at the distribution of how the notes are used, which is something I could see describing as valence. Plus, Major and Minor are binary, but there's really a continuum. Mary had a little Lamb is very very major. Moonlight sonata is very very minor. So I think here we're seeing songs growing increasingly less major. Specifically less major, though - there are other modalities of music outside of Major and Minor, and a single metric wouldn't really capture all of that - less major doesn't necessarily mean more minor, although we could generally assume it mostly goes that way.