A recent study of top-40 hits from 1965 through the first decade of the '00s shows that pop songs are increasingly sounding sadder since the mid '60s, i.e., being written in minor modes and containing more "self-focused and negative" lyrics. There's also been a trend toward longer and slower songs in charts. After my recent dalliance with the Billboard top 10, I'm inclined to agree with this analysis; I definitely left the experience feeling sadder for my ears.
Here's the abstract from the study Emotional Cues in American Popular Music: Five Decades of the Top 40 by Schellenberg, E. Glenn; von Scheve, Christian Psychology of Aesthetics, Creativity, and the Arts, May 21 , 2012:
1. Some musical characteristics are cues to happiness (fast tempo, major mode); others are cues to sadness (slow tempo, minor mode). Listening to music with inconsistent emotional cues leads to mixed feelings and perceptions, or simultaneous happy and sad responding. We examined whether emotional cues in American popular music have changed over time, predicting that music has become progressively more sad-sounding and emotionally ambiguous. Our sample comprised over 1,000 Top 40 recordings from 25 years spanning five decades. Over the years, popular recordings became longer in duration and the proportion of female artists increased. In line with our principal hypotheses, there was also an increase in the use of minor mode and a decrease in average tempo, confirming that popular music became more sad-sounding over time. Decreases in tempo were also more pronounced for songs in major than in minor mode, highlighting a progressive increase of mixed emotional cues in popular music. (PsycINFO Database Record (c) 2012 APA, all rights reserved)