There lies a kind of obligatoriness in Fiqh and Shari'a rules and scholars have dispute as to the origin of such obligatoriness. Some consider wisdom as the origin and others like some Ash'arites consider God's command as the origin and others consider human nature (al- Fitrah) as the origin. It seems that none of the three ideas are perfect. This paper regards the reason's grasp of obligation and necessity of obeying God as the root for Fiqh and Shari'a rules obligatoriness.