In chapter 8 of Lord of the Flies, why does Simon want to climb the mountain?
Unlike the other boys on the island, Simon understands the true nature of the beast and does not believe that there is some enigmatic, malevolent creature roaming the island. Since Simon is not worried about a beast attacking him, he has no problem climbing to the top of the mountain. It is also important to remember that Simon has already traveled through the forest alone at night without being attacked. In chapter 8, Jack attempts to usurp power and leaves Ralph's group. Shortly after, Ralph and Piggy seem discouraged and don't know what to do next. Simon then interrupts and says,
"I think we ought to climb the mountain." (99)
Piggy and Ralph dismiss Simon's suggestion and proceed to collect firewood while Simon heads to his secret spot in the forest. Later on in the chapter, Simon hallucinates, and the Lord of the Flies speaks to him. The severed pig's head confirms Simon's suspicions that the beast is the inherent evil inside each boy on the island. The Lord of the Flies tells Simon,
"Fancy thinking the Beast was something you could hunt and kill!" said the head. . . "You knew, didn't you? I'm part of you? Close, close, close! I'm the reason why it's no go? Why things are what they are?" (111)
In the next chapter, Simon wakes up with a clear head and decides to climb the mountain by himself to confirm his beliefs. In chapter 9, Simon discovers that the beast is actually a dead paratrooper. Unfortunately, Simon is mistaken as the beast during a severe storm and murdered before he can deliver the news.
In Chapter Eight, "Gift for the Darkness" after Jack splits from Ralph's tribe, Simon suggests that the remaining boys "ought to climb the mountain. [...] What else is there to do?" (128)
The rest of the circle, Piggy included, view Simon with "derisive incomprehension," and Simon's idea is quickly forgotten by the rest of the tribe (128). Simon, as suggested by his "what else is there to do" comment chooses to climb the mountain by himself. His actions are motivated by two desires: 1) a desire for action, over inaction, and 2) his curiousity to figure out what the beast truly is. The narrator reveals that Simon goes and sits far away from the others after his idea is vocally rebuffed by Piggy; this must hurt Simon, especially when he was one of the only boys to be consistently nice to Piggy.
Simon goes and climbs the mountain to do something; his decision to climb it by himself after the other boys declined reveals that he does not want to just sit around and wait for something to happen, and his decision might have also been a way to blow off steam after the other boys would not listen to his idea. Instead, Simon chooses to be proactive and go to the mountain to see for himself. Simon frequently liked to go into the jungle by himself to think and be alone. His solitary trip to the mountain will give him plenty of opportunity to do both.