Being a huge Harry Potter fan, there is something nice about having J.K. Rowling fill in some missing details. With the Potter universe being as big as it is, and detailed, Rowling provides additional info with the site Pottermore. The site also told me that my Animagus is a Rottweiler. For those of you who read my John Wick Pit Bull article, you'd know that I couldn't have been happier with the choice of animal. But I digress...
While having some information supplied by the author herself is cool, it does take away from some theories that fans might not want answered. Such is the case for why the Dursley family immediately hated and mistreated Harry Potter. Obviously taking on another child unexpectedly can cause some resentment, that wasn't it. The original fan theory came down to Aunt Petunia. The sister of Lily Potter can be considered a "Squib", a Muggle that was born to a wizarding family. Sounds more like shit luck, save for the fact that Aunt Petunia seemed to both admire and fear magic when she was younger. As she grew up, she saw the gifts of the wizarding community to be comparable to being a freak. So, not the best way to look at it. With that mindset, readers were left to assume she had no magical abilities herself, and was therefore a jealous Squib.
That is, until an interview back in 2004 where J.K. Rowling let it slip that Aunt Petunia was definitely not a Squib. How could this be?! It seemed like the author planned more story for Lily's sister, a storyline she seems to have abandoned. If the slip was true, Aunt Petunia does have some magical ability that she kept hidden. The theory took it a bit further, explaining that it was Aunt Petunia and not Harry Potter that caused the glass to shatter in Aunt Marge's hand in Prisoner of Azkaban. Readers just assumed it was Harry. Just in case you forgot, but the glass exploded when Aunt Marge began calling Lily names.
A great theory, right? Something I can only wish had evolved in the books. It also explains why Aunt Petunia was immediately hateful of Harry, as he represented another example of the world she was afraid of. Right?
Dursleys' Disliked James Before Harry
Before I jump into J.K. Rowling's explanation on why the Dursleys disliked Harry, might as well mention another theory that also made sense. In Harry Potter and the Deathly Hallows we learn that Harry is himself a Horcrux. A Horcrux is the darkest of magic, and that darkness could have an effect on Muggles if exposed to that dark magic over an extended period of time. For the Dursley family, their negativity could be explained by the fact that they were constantly around Harry. A good theory, but also wrong.
It turns out that the Dursley family disliked Harry Potter for the same reason Professor Snape did. He reminded them of James Potter. But, unlike the bullying that Snape experienced, Vernon and Petunia's dislike came down to a single dinner. According to J.K. Rowling, Petunia worked with Vernon and had an enormous crush on him. Hating her magical background, she was relieved to hear the following.
Dursleys Split from Lily and James
"He told Petunia solemnly that he would never hold it against her that she had a freak for a sister, and Petunia threw herself upon him in such violent gratitude that he dropped his battered sausage."
Vernon did give Petunia's relatives a chance, however. The newly-coupled pair went on a double date with Lily and James Potter, who were also dating at the time. The dinner went south when Uncle Vernon implied that wizards were on welfare. As we know, James Potter had money, and the wizard had no problem bragging about his gold. The night ended and so did Lily and Petunia's relationship. The only contact between the two after that was the announcement of Harry's birth and the announcement of Lily's death.
Vernon and Petunia especially hated James, and Harry looks a lot like James.
So there you have it! We still will always wonder what Petunia had wanted to say to Potter as she delayed her departure in Deathly Hallows. J.K. Rowling decided to leave her as one of the most hated characters. But still nowhere near Dolores Umbridge.