Jack and Ralph are two of the older boys in Lord of the Flies by William Golding, and they are the two clear leaders on the island. In chapter one, the boys elect Ralph to be their leader instead of Jack. While at first Jack is embarrassed about losing, Ralph quickly offers him the chance to be the leader of the choir, and Jack quickly claims them as his hunters.
Once that is decided, the boys are even a bit friendly.
Jack and Ralph smiled at each other with shy liking. The rest began to talk eagerly.
When Ralph decides to explore the island, he explains that it is important to determine whether this is truly an island so they can make a more effective rescue plan. He says:
Three of us—if we takemore we’d get all mixed, and lose each other—three of us will go on an expedition and ﬁnd out. I’ll go, and Jack, and, and. . . .”
Your question implies that perhaps Ralph is a willing participant in Jack's expedition up the mountain; in fact, it is Ralph's expedition and he invites Jack (and eventually Simon) to join him because Jack has already been exploring and, so far, the two boys are somewhat friendly.