It all started a long time ago...

"I really don't think they should let the other sort in, do you? They're just not the same, they've never been brought up to know our ways. Some of them have never even heard of Hogwarts until they get the letter, I imagine. I think they should keep it in the old wizarding families."

"You’ll soon find out that some wizarding families are much better than others, Potter. You don’t want to go making friends with the wrong sort. I can help you there."