I think what everyone who has posted is trying to say is that.......you will write or come up with your best music when its not forced.......Its not just being angry or happy or whatever, its the fact that you incorporate the sentiments into the music, and that its noticable.... As long as the emotions are not forced then the piece will be better...
I guess what I'm trying to say is that Emotions work to your advantage only when they are REAL!