Many reasons; but rather then me telling you why I personally think it's nasty, let’s look at what the Apostle Paul says about it.
1) Legalism is dangerous because it is and outward attempt to look spiritual. Galatians 2:6 “…God does not judge by external appearances.”
2) Legalism makes the death and life of Jesus meaningless. Galatians 2:21 “I do not set aside the grace of God, for if *righteousness could be gained through the law, Christ died for nothing!”
*Notice Paul uses the word “righteousness” here… if living right could get us in God’s good graces, then who needs Jesus. But, when you do accept God’s grace through Jesus you ARE righteous, which then leads you to right living.
3) Legalism holds us in chains… its bondage. Galatians 3:23 “Before this faith came, we were held prisoners by the law, locked up until faith should be revealed.”
4) Legalism actually draws us AWAY from Christ, and His grace. Galatians 5:4 “You who are trying to be justified by law have been alienated from Christ; you have fallen away from grace.”
I will continue this Legalism vs. Grace later...