Let me expand on literary reasons for those who don't have an interest in literature. Harry Potter is at its core an escapist book, much like the better known romance novels. Romance novels require for literary reasons a love interest and something keeping the two would be lovers apart, that's the core of the genre. Escapist books require something to escape from, some form of oppression and a fantasy, an alternate reality like escape for the character. That's why the Dursleys are there, whether you believe JK Rowling made the situation believable is a question about her skill as a writer. I think she did well but could have done better, like I said it doesn't seem to gel completely. That's what literary reasons mean, not "it's in a book it doesn't have to make sense" but that the essence of the story is based around some simple concepts and that those concepts are required to make a good story.
@siaru, take a creative writing course, or even join a book club, it'll do you good.