The answer to this question can be found in Chapter Seven, which explains John's penchant for visiting the nearest cemetery. As he sits there and wonders about if there is anybody in heaven and then anybody down below who wants to communicate to him, John suddenly realises that he is not really concerned about those above or benath him, but actually he is filled with a desperate uncertainty about what will happen to him after he dies:
Then I got very sad because I knew I wasn't really wondering about the guy underneath me, whoever he was. I was just interested in what was going to happen to me. I think that's probably the real reason I go to the graveyard. I'm not afraid of seeing ghosts. I think I'm really looking for ghosts. I want to see them. I'm looking for anything to prove that when I drop dead there's a chance I'll be doing something a little more exciting than decaying.
John then is filled with a kind of desperate uncertainty when he thinks about death. He hopes to find any form of tangible proof that after he dies he will have some kind of existence that is more meaningful than "decaying." Clearly, the thought of having his identity and his character obliterated by death is something that makes John very afraid.