If we fast, it should be because the Spirit leads us to fast. We wouldn't just decide "I'm going to fast because I think God would like this." THAT is a works-based decision. If that is the case, we are shortcutting a Spirit-led act. I liken it to my recent decision to become more involved in my Church. I felt like the Lord was leading me to really get involved more in the activities of my church. That meant doing more than just going every week with my wife and kids. We joined a Sunday School class that is involved in service projects around the community. Well, if I decided to skip Sunday School a couple of times a month or miss the service projects because I wanted to play golf or something, I'm choosing not to follow the leading of the Spirit. I don't think that God is going to get "mad" at me. But...I do think that He has a purpose for where He is leading me and that I can't be fully in His Will if I don't give Him my all in that.