Why do Ralph and Jack fight again after their reconciliation?

While the other boys are afraid to traverse the walkway and explore the caves, Ralph accomplishes the feat and is encouraged by his own bravery. He enters one of the caves and is soon joined by Jack. The two experience a brief reconciliation as they have fun together exploring the new mountain territory.

They continue along a narrow wall of rocks that forms a bridge between parts of the island, reaching the open sea. At this point, however, some of the boys get distracted and spend time rolling rocks around the bridge. Ralph again gets frustrated and then asserts that it would be better to climb the mountain and rekindle the fire. He accuses the boys of losing sight of their original goal, finding and killing the beast. Contradicting Ralph, Jack states that he wishes to stay where they are because they can build a fort.