Because he knows that they will be rescued.
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Simon suggests that Ralph should hold another meeting to tell the little-uns (little boys) Pg.51
Simon volunteers to run through the forest to tell Piggy that the others won't be back until after dark. Simon doesn't believe that the beast is a physical entity that could harm him, so he knows that there is nothing to fear in the forest.
When Simon discovers that 'the beast' which Samneric had seen on the mounatin top is simply the dead body of an airman he untangles the parachute lines which have caused the body to move like a puppet when the wind blows.
When Ralph asked for a volunteer to go and tell Piggy and the littluns that the biguns would be delayed Simon said, and I quote... "I'll go if you like. I don't mind, honestly." Then he smiled and ran off before Ralph had a chance to reply.
Ralph says that someone should go tell Piggy that they'll be back late, and Jack--the mean guy in the story--gets ticked off at Ralph and says it isn't necessary. But then Simon, who is actually a pretty nice character, volunteers to tell Piggy and runs off before Jack could say anything.