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.