Complementary signifies a unique difference and quality fit that makes one, the same signifies a misfit as with all relationships.  The word relationship means "one",  if each is the same they can not become "one". This is true for all relationships.  Consider how two pieces of pipe become one or the basic components of life.  Water is developed by the proper relationship of oxygen and hydrogen. A misunderstanding of this principle can be explosive and destructive for any relationship.  If the oxygen spouse tries to make the hydrogen spouse to be as they no water will result.