Why do they have conflict between Ralph and Jack?

In terms of greater significance, Ralph and Jack each represent different facets of human society (and perhaps human nature within each of us). Ralph is committed to civilization - justice and order - whereas Jack is committed to a "state of nature," where each person looks out for himself and there are no rules. These elements battle themselves out in society all the time, so it's natural it would happen here. In terms of more concrete character, forget not that they're boys. Jack's a less upright person, and is resentful of Ralph's control, especially since he thinks their island adventure is a chance to go without rules. So he pushes against Ralph's authority, and Ralph naturally fights back.