J.K. rowling explains why the dursleys hated harry

Harry Potter author J.K. Rowling has taken to revealing new details about the characters in her books through the Pottermore site—because hundreds upon hundreds of printed pages just cannot contain her genius—and she’s finally weighed in on a long debated topic: why the Dursleys hate Harry Potter so much. Sadly, it has nothing to do with that Horcrux theory.

If you’re the kind of fan who prefers to find the new info for themselves, you can head to Pottermore to click around a new interactive Deathly Hallows chapter and find out the new details for yourself, which include not only the reason the Dursley’s hated Harry but other tidbits about Dumbledore and Hagrid and what they meant to Harry.

As for the Dursleys, a convincing fan theory held that, because Horcruxes can influence those around them and Voldemort had made one out of Harry, the Dursleys’ dislike of him was exacerbated by Voldemort’s magic. But Rowling states that as for Vernon Dursley—true to form—he hated Harry for much more human reasons: he felt embarrassed by his father, James Potter, who Harry uncomfortably reminded him of.

so, if your theory was they were petty & jealous, you were right.