To Serve and To Praise 2

Rambam continues his elaboration of the principle that Hashem is the only One Whom it is proper to serve and to praise by contrasting Hashem with natural forces and celestial beings, whom it is improper to serve and praise because they do not act with free will, but are completely dependent on the will of Hashem.  Is Hashem the only One Who acts independently?  What about human beings?  Humans have free will; is it then permitted to serve them?  And why didn't Rambam contrast service of Hashem with service of human beings?