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Piggy was […] so full of pride in his contribution to the good of society […] that he helped to fetch wood. (8.118)
Ugh, Piggy, you're breaking our hearts. He's basically the only person who seems to care about the "good of society," so naturally he ends up dead. Without people who care about the common good, you don't have much of a civilization.
[Ralph] was vexed to find how little he thought like a grownup and sighed again. The island was getting worse and worse. (8.243)
This has got to be frustrating. Ralph knows that there's a "grownup" way to think, and he knows that he's not doing it. Again—Ralph might be surprised to find out that grownups feel this way a lot of the time, too.
"I dunno, Ralph. We just got to go on, that's all. That's what grownups would do." (8.260)
It's too bad that Piggy realizes this not too long before he gets murdered. There's no secret to acting like a grownup; it's just doing one thing after another, trying to stay ahead of the bills and the plumbing problems. (Seriously.)