Rolling friction is very common in our real life, but most students lack a correct understanding of the cause of rolling friction. In this paper, the cause of rolling friction is analyzed in two cases: a rigid body on a horizontal soft underlay and a soft body on a horizontal rigid underlay. It is found that rolling friction is a kind of effect caused by the asymmetric distribution of the force applied to the rolling body that hinders rolling when one object is rolling on another object or has a rolling tendency. The concepts of rolling friction and the force of rolling friction are also explained.