Why does Ralph ask Jack why he hates him?

because he feels that keeping the fire going is far more important than hunting. Jack has the desire to rule and Ralph stands in his way.

Jack, even in the beginning of the book, Jack wants to be chief. He nominates himself but is rejected as their leader in favor of Ralph, which sparks the flame of hate. Ralph does stand in his way but it's more that Ralph, for no honestly valid reason, was picked instead of Jack. On the mountain when they saw the Beast (the parachuter, in actuality), Ralph stops, leading Jack further to believe that Ralph is a coward and doesn't deserve to be chief. He sees it as unfair, which angers him and makes him hate Ralph for getting what probably seems like benefits when he "hasn't earned them."

Later, even when Ralph begins to lose power and Jack proposes that he replace the "chief", he is still rejected. This brings him to tears and angers him so much that he decided to make his own separate community. Even then, others still chose to stay with Ralph. No matter how hard Jack tried, Ralph always seemed to be more able to win others over, leaving Jack with very little power, especially compared to his position at the very beginning of the book as leader of the choir boys. He was always obeyed by them, and didn't like the change.

Essentially, he hated Ralph because he saw the position Ralph had as undeserved and unfair. It felt to him like he'd been stripped of his position and replaced with someone who he thought was less capable. It made him hate Ralph.