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    A smooth surface reflects light more in one direction.  A rough surface scatters light in all directions.  Then the intensity of light received in one direction is quite less.

     A dull or rough surface when seen through a large magnifying lens looks like a large ground with a lot of stones, crevasses, cliffs, valleys, cuts, scratches, sharp edges, some projections protruding out of the surface.

   The wavelength and amplitude of light are very small.  So when they hit the various parts of the surface, they get reflected in various directions that are normal to the surface at the point of hitting the surface.   So the rays are reflected in random directions.  The amount of light that is reflected by the surface and reaching our eyes is quite less.
       So we perceive the surface to be dull or rough.

   A smooth surface does not have cracks on that.  There are no valleys and projections.  So the surface is mostly along a single straight line when viewed with a magnifying lens.   Thus the reflected light is always in one direction.

   The light rays are reflected more than 90% from the surface and reach the eyes.  So a smooth surface reflects light a lot and shines bright and brilliant.
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Because shiny surfaces are smooth.They reflect light rays in the parallel direction.Whereas in rough surfaces there is irregular reflection at any angles.