Why the coefficient of static friction usually is greater than kinetic friction?

Question

Why the coefficient of static friction usually is greater than kinetic friction?

Answer 1

It helps to think of surfaces not as smooth, but as rough with many hills and valleys.  When the objects are still (relative to each other), these hills and valleys can line up and maximize the force between them.  When the objects are moving, the hills and valleys don't get a chance to line up, thus it takes less force to keep the object moving than it does to make it start moving.  The alignment of the hills and valleys is rarely perfect.  When it is, objects cold weld together and you really can't pull them apart because they fuse into one.  This phenomenon is seen with highly smooth metal surfaces.

Answer 2

It is usually more due to inertia, which is an object's resistance to change in motion.