The Sorting Hat’s decision is not always black and white. Both Ron and Harry are contenders for Slytherin—Ron is a pureblood, and on Harry’s first day at Hogwarts, detailed in Book I, the Hat notes that Harry would do very well in Slytherin, but Harry, mumbling under his breath, manages to convince the Hat to place him in Gryffindor. Meanwhile, Hermione seems like the ideal candidate for Ravenclaw, with her stunning intelligence and uncompromising study habits. Still, the Hat places all three friends in Gryffindor, which speaks well to their inherent bravery. Time and time again, Ron, Hermione, and Harry prove they are courageous and strong. Neville Longbottom, who initially seems like an unlikely candidate for Gryffindor, has also proved his worth, displaying unexpected bravery and loyalty. The Hat’s decisions often seem mysterious, but so far, they’ve turned out to be exactly right, a fact that makes the eerie song it sings this time even more ominous.
Most of the inter-House competition is healthy and fair, but the rivarlry between Gryffindor and Slytherin frequently crosses the line into blatant hatred. Their deeply rooted rivalry shines through clearly in their Quidditch matches, which seem to take on more weight than any other match-up. Even the two Houses’ position at the top of the Hogwarts crest (facing each other, poised for battle) seems to suggest a certain innate rivalry. Likewise, Harry, Ron, and Hermione, the main Gryffindor representatives, are clearly more offended by Draco Malfoy and his goons, the main Slytherin representatives, than by any other group of students. Harry’s least favorite professor is Snape, Head of House for Slytherin, and he clearly respects and enjoys Professor McGonagall, Head of House for Gryffindor, more than any other faculty member besides Dumbledore. While general competition can be healthy and productive for students, this kind of specific, one-on-one battle breeds danger and discontent.
The internal splintering happening at Hogwarts helps Voldemort in that it breeds tension and malcontent among students and faculty, weakening their defenses against Dark imposters. As Nearly Headless Nick explains, the Sorting Hat warns students about the dangers of the House system only when it feels the school is in danger. The House system is not the only thing that encourages distrust among the students. All summer, the Daily Prophet has been printing unfair and untruthful stories about Harry and Dumbledore, portraying them as foolish and arrogant. Many of the students at Hogwarts regularly read the Prophet and believe what it says, and they treat Harry with distrust and apprehension. His peers and classmates regularly gossip together in the hallways, pointing and whispering about Harry. Like any normal fifteen-year-old boy, Harry is embarrassed and disappointed by all the unwanted attention. That Harry’s classmates do not believe Harry or even Dumbledore, their trusted Headmaster, is disheartening. Already, Hogwarts is displaying strong signs of internal dissolution.