Rules tend to promote self righteous, works based people who "earn" their salvation by doing the "to do list". If I do everything on the list I am a good person! That makes us feel comfortable and secure in our salvation. We don't need the help of God's grace - we've checked off all the requirements and we're good!
Being called to Holiness, however, shows us one thing - we are sinners! When I am pointed to God and what HE says is right and wrong and I am longing with all my heart to do what is right to show Him my love, it doesn't take long to realize there is NO WAY I can ever get it right!! It is only by His Grace that I can be saved. And, just in the same way that Chris looks at me with love the night he comes home and the baby has been screaming and the three year old cut their finger and supper simply is not ready and says "Its ok. I know your heart and that you wanted to have supper ready on time." So God looks at me when I fail and says, "I know your heart, I know you love me and really want to obey me. Its ok."