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.