Jack, a tall, strong-willed, forceful ego maniac takes an immediate dislike to Piggy.
"Piggy is a man of thought, not of action, and he is physically weak because of his asthma."
When Jack looks at Piggy, he sees everything that a typical bully loves to pick on, an overweight kid, with glasses, who doesn't like to fight and can barely defend himself against an insult.
From the start, Piggy is fearful of Jack, who after he strips away his last sense of civilized propriety, oozes with savagery. He is frightening to be around, and Piggy makes sure to stick with Ralph.
Jack tolerates Piggy in the beginning, but his clinging to the rules of civilized behavior start to irritate Jack as the novel progresses.
As Jack becomes more detached from any form of social order, and assumes a primal nature, he and Ralph begin to fight. The disputes encourage Piggy to insist on rules all the more. This further separates Piggy from the group and ultimately the disconnect between the boys results in the death of Piggy.
Jack and Piggy represent opposite forces in the book and thus they are at odds from the beginning. Piggy represents civilization and the world of thought and reason. He is fat, ugly, soft, and weak. His strength is in his ability to reason. It is he who thinks of building the fire and his glasses that start the fire. His glasses further represent civilized thought. Piggy is not a leader; he needs Ralph to put forth and convince the other boys of his ideas. He is everything that Jack is not.
Jack represents the savage world of the island, and he despised what he sees as Piggy's weakness. Jack is also smart and he is a leader. The reader first sees him leading his fellow choir members as if they were a military formation. Jack is the first one to try to hunt on the island and he soon develops a taste for the blood of the hunt. Jack further separates himself from civilization by donning a mask and he becomes increasingly animalistic as the book continues leading up to Simon's death. A key moment is when Jack steals Piggy's glasses because the glasses represent civilization and by taking them from Piggy he demonstrates the strength of the savage nature of man over the veneer of civilization and reason.