In Lord of the Flies, Ralph undergoes a profound change of personality during the island stay. Can someone give me an explanation?

Ralph begins the novel disdainful of Piggy, interested in fun and games, joyful at the prospect of being on a tropical island.  Throughout the novel, Ralph matures more than any other character.  He learns to appreciate Piggy's value, that Piggy has the ability to think.  He learns that being a leader on the island is not all fun and games, that it is a huge responsibility, and that mistakes on his part can mean disaster.

The event that is pivotal to Ralph's development is his spotting of the ship when the signal fire has gone out.  Jack and his group who were in charge of the fire neglected it to hunt and kill their first pig.  Ralph realizes here how desperately he wants to be rescued and the need to establish order on the island.

He immediately calls another meeting to set things straight.  In this meeting we see Ralph clearly developing into a good leader.  He realizes he has to say things twice; he knows that there must be certain rules laid down and respected if the boys are to live together and be rescued; and he also knows that he must somehow address this boys' fears.

Ralph's inability to address the boys' fears of the beastie results in his decline as a leader.  His responsibility and failure weighs heavily on him.  He is the only boy who admits that they murdered Simon, for instance.  Piggy, Sam, and Eric deny taking part in Simon's murder, but Ralph is honest.  Ralph longs for civilization and regrets the savagery that has overtaken the boys, himself included.  He understands the need for rules, law and order.

At the end of the novel, Ralph leans another lesson:  he must think like a pig in order to survive.  When chaos breaks out on the island, and Ralph is the object of the massive manhunt, Ralph must become like a pig--acting instinctively to avoid the rocks and sticks that Jack's tribe is using against him.

In the novel's closing, Ralph weeps for the "end of innocence, the darkness of man's heart, and the fall through the air of the wise, true friend called Piggy."  Ralph has witnessed the dark side of man's nature--in himself as well as others--and will never be the child he once was who thought his father could rescue him from his troubles.  He feels tremendous guilt and sadness.  He comes away with the knowledge that leadership is a very difficult undertaking and a huge responsibility and the consequences of failure are fatal.