Survey on Emotions

To be honest, I'm not sure where I stand on the issue of emotions in the Christian life. Don't get me wrong, I believe that they exist and that they're important. I am just having difficulty figuring out how I should think about them. Some of my friends say that they are demanded by God and so emotions are commands. I can see their point. Other friends of mine say that commands to "Rejoice in the Lord always" are volitional choices we make which are followed by emotions. They point out that we cannot conjure good emotions when we feel bad. And since God expects it of us, it cannot be unattainable. I am reading "When I Don't Desire God: How to Fight for Joy" by John Piper to help get more insight. What are your thoughts on this subject? Where would you go in the bible to point out what you believe? How would you answer my friends who don't think emotions are what God is commanding?