Why can't Ralph understand why Jack hates him ?
Because Ralph isn't wired like Jack is. He isn't threatened by Jack's leadership skills as Jack is by Ralph's. That is the difference in their ability to motivate others to follow. Ralph's is by reason, logic, and fair play. He attempts to treat others as he wants to be treated and to live with ease and a sense of duty to community. In other words, you do your job because others are depending on you...ie, don't let the fire go out, collect fresh water, etc.
Jack, on the other hand, rules by fear and intimidation. He hates Ralph because Jack does not command the same respect that Ralph does. Jack only has followers because the boys don't want to be beaten or abused, and because Jack provides hot meat to eat. He gives them two of the basic needs--food and "safety" from the beast.
Also, Ralph never wanted to be a leader. He had the job thrust upon him by the actions of others, particularly Piggy. Therefore, he can't understand Jack's jealousy because he himself would never have that jealousy.