The academic journal Psychology of Popular Media Culture reported on some research by professors at the University of Michigan at Dearborn. Pam McAuslan and Marie Waung examined pop music charting in 1990 , 2000 and 2010 and found that over time, it has gotten a lot more focused promoting the self and demanding respect.
Bragging was confined mostly to rap music in the 1990 survey -- which makes sense, as a significant feature of earlier rap and hip-hop music was to demonstrate skill at wordplay. Rappers claimed superiority over others by virtue of their ability to make a clever rhyme or successfully unleash a tongue-twisting torrent of words at high speed without any missteps.
But during the next surveys, of 2000 and 2010 top hits, McAuslan and Waung found that songs in a variety of genres were about the singer's self-regard. Bragging expanded to sexual conquests, and the frequency of third-person references to the self increased. There were simply many many more songs that went ahead and lived out the Toby Keith number, "I Wanna Talk About Me."
The increasing complaints about how today's teens and twenty-somethings have been trained to think of themselves as the center of the blinkin' universe may have something to do with it, but I don't know for certain.