Only two objects do not go into retrograde motion; The Sun and Moon.
http://astrologyclub.org/why-planets-A great explanation of retrogradation is seen in the link below with illustrations;
http://astrologyclub.org/why-planets-retrograde/Retrogradation is a phenomenon caused by the fact that we live on a planet which is not at the center of the solar system. If one were to view the solar system from the surface of the Sun, which is at the center, one would never see planets retrograde. However, since that isn’t where we’re viewing things from, we do see planets appear to stop their motion in the sky relative to the fixed stars (or the Zodiac), reverse their direction for a little while, then stop again and appear to resume their normal course through the heavens.