15. what does the story suggest the cause is for Young Goodman Brown's situation at the end of the story?

At the very end of the story, Young (I guess he's not young anymore ) Goodman Brown dies. He is followed to his grave by Faith and by their kids and their grandkids. When they bury him in his grave,

there is no hopeful verse upon his tombstone, for his dying hour was gloom.

If you are asking about the part a bit before that, what happens is that what he sees in the forest makes him lose his faith and his hope. He thinks everyone is corrupt and evil, so he has no good thoughts about anyone. We don't know whether what he saw in the forest really happened, but it doesn't matter. He acts as if it does and it ruins his life.