Why does Orlick hate Pip so much?

There are a few reasons. Orlick hoped to be Joes apprentice.

"When I became Joe's 'prentice, Orlick was perhaps confirmed in some suspicion that I should displace him."

Orlick is also convinced that it was Pip who ruined his chances of falling in love with Biddy: "it was you as always give Old Orlick a bad name to her."