In "By the Waters of Babylon" why does John set out on his journey?

There are a couple reasons that John takes his journey.  The first relates to the traditions of his tribe.  He is the son of a priest, and all priests, kind-of like some Native American tribes or Aboriginal tribes in Australia, have their sons go on a "spirit walk."  This is where a boy, who is approaching manhood, sets out on his own, to survive by himself, in search of revelation, wisdom, meaning, and tests that will prove that he is a man.  It is a rite of passage, one that will be a step in achieving manhood in the eyes of the tribe itself.  Nowadays, we don't have such extreme rituals--maybe getting your driver's license, or getting your first salaried job would be akin to the same idea--once you've achieved that accomplishment, you can call yourself an adult.  So, that is the first reason that John goes--it is tradition, a ritual in his tribe, and he sets out in search of his wisdom, knowledge, or trials that will prove his strength.

The second reason that John goes is because he has a very intense desire for knowledge.  He is a curious guy, one who wants to know the history of the gods, of his people, and why the world is the way it is.  His desire drives him to his journey, because it filled his dreams.  The dreams drive him closer and closer to the city of the gods.  He wants to know, and despite such journeys being forbidden, he states that "the burning in my mind would not let me have peace."  His burning desire for knowledge drives him forward on his journey, breaking rules and boundaries, and the result is that he does indeed gain great knowledge.  He learns of their origins, that the gos are "men, like us."  He is able to bring that knowledge back to his tribe to benefit them all.  I hope that those thoughts helped; good luck!