This is one of my favorite Hank Williams songs: http://www.youtube.com/watch?v=nzY2Qcu5i2A&ob=av2e
Listen to it, and note that the music is not sad at all. The LYRICS are sad.
This is a two channel communication. On one channel you have happiness and on the other sadness. Our conscious minds process the sadness, but temper it unconsciously with rhythm.
The task in sadness is to continue. The music does this. This song has always felt cathartic to me. Really, it makes me happy. I don’t know why, and this is especially true when I am already feeling down.
I think it has to do with the pattern interrupt implied by the dichotomy between the music and the words.