“Earth to earth, ashes to ashes, dust to dust” is a common religious metaphor that distinguishes between our physical bodies and our spiritual souls. Romeo is using that metaphor when directing his physical body, his "dull earth," to go back to Juliet’s house because that’s where his heart and soul are. Notice the clever pun he makes with the word “earth.” He’s drawn toward Juliet just as “earth,” or dirt, is drawn by gravity to its own center, the “Earth’s” center.