In John's society, so reminiscent of ancient and tribal societies that were precursors to our modern era, as a priest, he wants to go on a "spirit walk" to find knowledge, and as an initiation into manhood. In similar societies, many young boys, who are growing older, are expected to face initiation rites that usher the boy into manhood. John, as the son of a priest, has learned a lot, but states,
"nevertheless, my knowledge and my lack of knowledge burned in me -- I wished to know more. When I was a man at last, I came to my father and said, 'It is time for me to go on my journey. Give me your leave.'"
So, he has been trained by his father in spells, chants and wisdom, but he wanted to know more. That is the initial reason that he starts out on his journey--to gain further knowledge for himself, as a man. However, when he gets to the actual boundaries of the place of the gods, he could have stopped. It had been forbidden to go there, so maybe he should have. But, John desires to enter into the city because
"If I went to the Place of the Gods, I would surely die, but, if I did not go, I could never be at peace with my spirit again."
He had a burning desire in his spirit--his spirit was prompting him onward, to learn things, to conquer that obstacle, and he decided that he would regret it for the rest of his life if he didn't go. He would always have that burning inside of him, that curiosity, and that regret. He states as he enters within that his "hunger for knowledge" drove him on. So, he goes, and discovers quite a bit that he can bring back to his tribe that will help them to progress.
I hope that those thoughts helped a bit; good luck!
He enters the place of gods in search of knowlege.