As you may recall, Harry was sent to live with his deceased mother’s sister, Aunt Petunia and her family, Uncle Vernon and Harry‘s cousin, Dudley. At the age of one, Harry had lost both his parents by the wand of Voldemort and somehow defeated the dark lord. Regardless of his magical fame, the ignorant Muggle Dursleys treated Harry like he was a disease and had no pity on him for having lost his parents at such a young age, not even from his aunt who lost a sister. The Dursleys were a bunch of miserable Muggles, but it was always so baffling how they could be so cold and cruel towards Harry Potter. But now the author, JK Rowling, set out a back story on Pottermore that reveals the true reason behind why the Dursleys hate Harry.
Rowling explains that the cruel treatment and resentment thrown at Harry was the result of Aunt Petunia‘s jealousy towards her sister, Lily because secretly Aunt Petunia wanted to have magical abilities as well. Uncle Vernon‘s hate comes from his ignorance and the fact that he hates anything weird or out of the ordinary. Kind of like an ignorance is bliss situation.
But the hatred dates back to when Uncle Vernon and James Potter (Lily‘s boyfriend at the time) met for the first time and had a fall out. During this meeting Uncle Vernon acted the only way he knows how – by being arrogant, shallow and, most of all, ignorant and intolerant of anything strange. Uncle Vernon tried to be the bigger, wealthier, more successful man, but James was always going to better than a lazy, narrow-minded Muggle. After Uncle Vernon‘s attempts to patronise James failed, he started to resent James for his success (and gold) in the magical world. So, as Rowling explains, Aunt Petunia resented her sister therefore Harry was punished for the jealousy she felt towards Lily. Uncle Vernon hates Harry because he resembles James so much and Uncle Vernon got OWNED by James.
Rowling clearly hated the Dursleys herself, explaining that the name Vernon was chosen because she hates the name and Petunia always represented “unpleasent female characters in games of make believe I played with my sister, Di, when we were very young“.
[The Dursleys are] reactionary, prejudiced, narrow-minded, ignorant and bigoted; most of my least favourite things.