Simon and Piggy are the two most reasonale and prescient characters in The Lord of the Flies, aware of the potency of the social dynamics on the island. Ralph, though not a perfect leader, represents a civility and a rationality of purpose that speaks to Simon's desire for safety and order.
Simon's desire for safety is underscored by his habitual escapes into the little den where he likes to hide. His reason and prescience are proven by his venture up the mountain, despite the voices in his head, to discover the true nature of the beast.
These impulses in Simon characterize him as a voice of resistance against the wild urges of Jack and his (eventual) group of savages.
In choosing to support Ralph over Jack, Simon is choosing safety and order over the wildness that he rightly sees in Jack.