I like this because now I have a study to back me up when I talk about sad music and nostalgia and stuff.
For me, It's simple - When I'm happy, I don't need music to represent my happiness. When I'm sad, I need not only commiseration, but also to feel that others have felt the way that I am feeling.
I was given a talk by Emury Schubert from the University of New South Wales. They found that there's also something that they've dubbed the 'affect gap'. Put simply, if the emotion triggered in you by the music is the same as what you perceive the music to be actually expressing, then you will derive enjoyment from it. So if the music makes you feel sad, and you perceive the music to expressing sadness, then you're more likely to appreciate it. You'll enjoy it less and less as the gap between those two categories grows, He also did a paper back in 2011 on why people enjoy negative emotion in music.