Up until chapter 7, why does Jack hate Ralph in Lord of the Flies?Their antagonism becomes more and more open, even though Ralph went through some...

Their competition begins, of course, that first day on the beach when Ralph is elected leader of the group.  Ralph quickly acknowledges Jack as another strong member of the group by giving him the responsibility of hunting and of keeping a fire going; but even this gesture could have been insulting to Jack if he chose to view it as an act of condescention. 

As the book progresses, Jack despises Ralph because Ralph constantly makes him confront his worst fear--that a rescue is not going to happen.  By hunting and neglecting the fire, Jack calls himself being practical and making the best of a terrible predicament.  However, Jack is ignoring his subconscious fears.  It's the same principle that occurs when, say, a child who is continually picked on chooses to become the class clown and joke about his/her own shortcomings.  Jack is covering up his fears of inadequacy and his fears of abandonment by the adult world by choosing to pretend they don't exist.  If he can encourage the other boys to follow his leadership, so much the better--there is confirmation in company.