“The pasuk states: ‘Any open vessel with no cover fastened to it is impure.’  Every human being is a vessel in the service of Hashem.  Thus, he requires a head-covering in order to control the level of outside influence that he allows inside.  The first thing a wayward Jew does r”l is take off his yarmulke to signify that he is ‘free’ of conforming to the will of Hashem.  A Jew must always control his negative impulses and appreciate the boundaries that our forbears have provided for us.” ~ Michtav Sofer (Torah Tavlin)