I believe it is because when you are in a bad mood, your emotions are more passionate. Think, when you are angry, don't you have thousands of things to say to the person who made you angry? Well, I know I do. The best written works are written with passion. When you are happy, often, it isn't anywhere near as passionate unless something exceptional has happened.
Same with me I can only write songs or poetry when I'm depressed, otherwise it comes out like junk, I can never seem to write about happy things
I find it easiest to write when I am drunk. I also write when I depressed mainly because my story is a depressing story.
Emotion drives you to write originally and that must be soemthing good :D
That happens to me too!!
im the same wierd