Ralph and Jack's relationship is extremely strained in Chapter Seven of Lord of the Flies. In chapter five, Piggy observes to Ralph about Jack:
"'He hates you too, Ralph--'
'Me? Why me?'
'I dunno. You got him over the fire; an' you're chief an' he isn't'" (93).
After Piggy's insightful comment, Ralph is still not sure that Jack really hates him, but their interactions on the mountain in chapter seven confirm Piggy's insight for Ralph.
In chapter seven, the older boys go into the jungle to search for the beast that Sam and Eric saw, and there have been minor moments when Jack and Ralph have been vying for the role of leader. Of course, Jack has wanted to be chief all along and resents Ralph as the chief. Jack has been antagonizing Ralph since they left the beach, making mocking comments in a "queer, tight voice" (117).
"Ralph looked back at Jack seeing him, infuriatingly, for the first time" (117).
Ralph realizes in this moment that all of Piggy's observations about Jack Merridew are true. Jack antagonizes Ralph because he wishes he were the chief. Ralph asks Jack, "Why do you hate me" (118)? Jack's lack of response confirms Ralph's belief.
The rest of the chapter, Jack attempts to outdo Ralph in bravery and leadership by guiding the boys up the mountain, using every opportunity to belittle Ralph as a leader.