I think it's just that you were more into the songs you learned when you were younger. I notice the same thing with movie plots. I can remember every detail of movies I liked when I was 14, but I mentioned to someone the other day that these days unless a movie makes a very strong impression on me, I forget much of the plot the moment I walk out of the theater or living room. Harry Potter films, for example. I remember enjoying them all, but apart from a few standout scenes I forget almost every plot, mainly because I don't care as much about it as much as I would if I were still 14.