Grover gets really angry when he's talking to Percy about the environment, just after they kill Medusa.  They trio have made camp for the night, and Grover looks around at all the garbage on the ground and laments the fact that there is so much air pollution that he cannot even see the stars.  He's angry at how much and how quickly humankind is ruining the environment, and he snaps at Percy when Percy acts nonchalant about it.  At a satyr, Grover is very connected to nature, and he also wants to become a searcher, a satyr who looks for Pan, the God of Wild Places, to wake him from his sleep.  The satyrs refuse to believe that Pan has died, although humans believed it long ago, and that made us feel like we had license to do whatever we wanted to the environment.  This carelessness really angers Grover.