Why do you think the boys believe in the natural goodness of man?
They believe in the natural goodness of man for two reasons. Firstly, they have not had the experience otherwise of men to have seen anything other than goodness. In their small world before the island, they have generally not experienced any hardships that would lead them to think otherwise. And secondly, they have been told by their parents, teachers and the church that man is good. Or at least that man has the ability to be good. For the boys good is anything which is, "virtuous...or confirms to the moral order of the universe."
This belief has never been tested before the boys are on the island.
In addition to the excellent answer above, I think another reason is human beings desire to believe that if they should ever be tested, that the positive aspects of human nature will "bleach out" if you will, the darker "nights of the soul." Unfortunately, this is not the case. Man seems to have a innate desire to survive first and ask moral questions later.