Pop songs are sadder than 50 years ago, why?

The use of words related to negative emotions, in English-language popular songs at least, has increased by more than one third in the last 50 years, an analysis of lyrics of more than 150,000 songs has shown. Let’s assume an average of 300 words per song, every year there are 30,000 words in the lyrics of the top-100 hits. In 1965, around 450 of these words were associated with negative emotions, whereas in 2015 their number was above 700, Aeon reports. Words associated with positive emotions decreased in the same time period - there were more than 1,750 positive-emotion words in the songs of 1965 and only around 1,150 in 2015. The tempo and the tonality of pop songs also changed - hits have become slower, and minor tonalities have become more frequent (minor tonalities are perceived as gloomier with respect to major tonalities). No definitive explanation, just assumptions - less centralised record industry, societal changes...