Why the Sorting Hat considered Harry for Slytherin is -- like the question of whether or not Harry was a Horcrux -- one of the most contentious questions in the Harry Potter fandom. Let's take a look at why the Sorting Hat considered Harry for Slytherin.

Some believe that Harry simply exhibits the traits the Sorting Hat looks for when placing a student in Slytherin -- ambition; achievement; cunning; cleverness; perseverance; and an insatiable desire for successful endeavors. Yes, J.K. Rowling writes, these are traits that Voldemort also particularly values. But you know? Many people value power; power is not a pejorative. Powerful people contribute to our communities every day, in infinitely unique and positive ways.
Others think, although Harry is a half-blood, that his Peverell lineage lends him to Slytherin House's tradition of taking students from long lines of witches and wizards, in other words pure-bloods.
Still more feel the Sorting Hat considered placing Harry into Slytherin because the Hat wasn't going to let anyone dictate its decisions, not even the Chosen One¹.
Our own lovely
ashindk says, "My head canon is that the Hat wanted to put Harry in Slytherin because he has a great deal of ambition towards being needed and wanted. And because he is a rule breaker."
All plausible theories, yes? Absolutely.
Let's review the canon passage and then compare it to the reason J.K. Rowling gives for why the Sorting Hat considered placing Harry in Slytherin:
‘Hmm,’ said a small voice in his ear. ‘Difficult. Very difficult. Plenty of courage, I see. Not a bad mind, either. There’s talent, oh my goodness, yes – and a nice thirst to prove yourself, now that’s interesting² ... So where shall I put you?’
Harry gripped the edges of the stool and thought, ‘Not Slytherin, not Slytherin.’
‘Not Slytherin, eh?’ said the small voice. ‘Are you sure? You could be great, you know, it’s all here in your head, and Slytherin will help you on the way to greatness, no doubt about that – no? Well, if you’re sure – better be GRYFFINDOR!’
Harry Potter and the Philosopher's Stone - pages 90 - 91 - Bloomsbury - chapter 7, The Sorting Hat
J.K. Rowling: "Also, I always imagine that the Sorting Hat detected the presence of that piece of [Voldemort's] soul when Harry first tried it on, because it's strongly tempted to put him in Slytherin."
POTTERCAST - INTERVIEW WITH J.K. ROWLING - 12.23.07 - THE LEAKY CAULDRON
Okay. So, what does this mean?
It means the Sorting Hat is an incredibly intricate magical item. Not only is the Sorting Hat sentient, but it's a Legilimens:
The Sorting Hat is one of the cleverest enchanted objects most witches and wizards will ever meet. It literally contains the intelligence of the four founders, can speak (through a rip near its brim) and is skilled at Legilimency, which enables it to look into the wearer’s head and divine his or her capabilities or mood. It can even respond to the thoughts of the wearer.
POTTERMORE - THE SORTING HAT
The Sorting Hat, during Harry's Sorting, sensed the piece of Voldemort's soul residing in Harry, and subsequently the Slytherin characteristics of Tom Riddle/Voldemort. This gave the Hat pause. If Harry had known, at eleven, that he carried a piece of the Dark Lord's soul inside him, the Sorting Hat, a Legilimens, could have conceivably talked with Harry about it and maybe even relayed helpful information to Dumbledore. The Sorting Hat seriously considered placing Harry in Slytherin because Harry had, at that time, the essence of Slytherin inside him. The Hat wasn't toying with Harry when it asked, "Are you sure?" The Hat saw Slytherin.
That said, the Sorting Hat placed Harry in Gryffindor.
ES³: Has the Sorting Hat ever been wrong?
JKR: No.
ES: Really?
JKR: Mm-mm.
A true Slytherin wouldn't ask to be placed in another house.
Our Harry is a true Gryffindor, through and through.
But, still. A girl can dream in silver and green ... :)

DID YOU KNOW?
¹Personally, I don't accept this at all. Canon has demonstrated the Sorting Hat takes a student's desire to go to a given house into consideration. As we know, Harry did go to Gryffindor, as opposed to "anything but Slytherin." The hat considered Ravenclaw for Hermione; Hermione had expressed in the chapter 6, The Journey from Platform 9¾ in Harry Potter and the Philosopher's Stone that she hoped to be Sorted to Gryffindor. In chapter 5, Diagon Alley, also in Philosopher's Stone, while they were getting their school robes fitted at Madam Malkin's, Draco Malfoy told Harry that he wanted to go to Slytherin. Conversely, Neville Longbottom argued with the Sorting Hat for almost five minutes about being Sorted to Gryffindor; Neville didn't feel he was suited for Gryffindor and attempted to convince the Sorting Hat to send him to Hufflepuff. In this instance, the Sorting Hat chose not to grant a student placement in his desired house. We'll discuss this further when we formally take a look at the Sorting Hat itself and the phenomenon known as a 'Hatstall'.
² The way I read this, this is the moment the Sorting Hat detected the presence of Tom Riddle/Voldemort's soul inside of Harry.
³Emerson Spartz of Mugglenet - INTERVIEW WITH J.K. ROWLING - PART 2 OF 3 - 7.16.05

GUH.
Some believe that Harry simply exhibits the traits the Sorting Hat looks for when placing a student in Slytherin -- ambition; achievement; cunning; cleverness; perseverance; and an insatiable desire for successful endeavors. Yes, J.K. Rowling writes, these are traits that Voldemort also particularly values. But you know? Many people value power; power is not a pejorative. Powerful people contribute to our communities every day, in infinitely unique and positive ways.
Others think, although Harry is a half-blood, that his Peverell lineage lends him to Slytherin House's tradition of taking students from long lines of witches and wizards, in other words pure-bloods.
Still more feel the Sorting Hat considered placing Harry into Slytherin because the Hat wasn't going to let anyone dictate its decisions, not even the Chosen One¹.
Our own lovely
All plausible theories, yes? Absolutely.
Let's review the canon passage and then compare it to the reason J.K. Rowling gives for why the Sorting Hat considered placing Harry in Slytherin:
‘Hmm,’ said a small voice in his ear. ‘Difficult. Very difficult. Plenty of courage, I see. Not a bad mind, either. There’s talent, oh my goodness, yes – and a nice thirst to prove yourself, now that’s interesting² ... So where shall I put you?’
Harry gripped the edges of the stool and thought, ‘Not Slytherin, not Slytherin.’
‘Not Slytherin, eh?’ said the small voice. ‘Are you sure? You could be great, you know, it’s all here in your head, and Slytherin will help you on the way to greatness, no doubt about that – no? Well, if you’re sure – better be GRYFFINDOR!’
Harry Potter and the Philosopher's Stone - pages 90 - 91 - Bloomsbury - chapter 7, The Sorting Hat
J.K. Rowling: "Also, I always imagine that the Sorting Hat detected the presence of that piece of [Voldemort's] soul when Harry first tried it on, because it's strongly tempted to put him in Slytherin."
POTTERCAST - INTERVIEW WITH J.K. ROWLING - 12.23.07 - THE LEAKY CAULDRON
Okay. So, what does this mean?
It means the Sorting Hat is an incredibly intricate magical item. Not only is the Sorting Hat sentient, but it's a Legilimens:
The Sorting Hat is one of the cleverest enchanted objects most witches and wizards will ever meet. It literally contains the intelligence of the four founders, can speak (through a rip near its brim) and is skilled at Legilimency, which enables it to look into the wearer’s head and divine his or her capabilities or mood. It can even respond to the thoughts of the wearer.
POTTERMORE - THE SORTING HAT
The Sorting Hat, during Harry's Sorting, sensed the piece of Voldemort's soul residing in Harry, and subsequently the Slytherin characteristics of Tom Riddle/Voldemort. This gave the Hat pause. If Harry had known, at eleven, that he carried a piece of the Dark Lord's soul inside him, the Sorting Hat, a Legilimens, could have conceivably talked with Harry about it and maybe even relayed helpful information to Dumbledore. The Sorting Hat seriously considered placing Harry in Slytherin because Harry had, at that time, the essence of Slytherin inside him. The Hat wasn't toying with Harry when it asked, "Are you sure?" The Hat saw Slytherin.
That said, the Sorting Hat placed Harry in Gryffindor.
ES³: Has the Sorting Hat ever been wrong?
JKR: No.
ES: Really?
JKR: Mm-mm.
A true Slytherin wouldn't ask to be placed in another house.
Our Harry is a true Gryffindor, through and through.
But, still. A girl can dream in silver and green ... :)

"GRYFFINDOR!"
DID YOU KNOW?
~*~
¹Personally, I don't accept this at all. Canon has demonstrated the Sorting Hat takes a student's desire to go to a given house into consideration. As we know, Harry did go to Gryffindor, as opposed to "anything but Slytherin." The hat considered Ravenclaw for Hermione; Hermione had expressed in the chapter 6, The Journey from Platform 9¾ in Harry Potter and the Philosopher's Stone that she hoped to be Sorted to Gryffindor. In chapter 5, Diagon Alley, also in Philosopher's Stone, while they were getting their school robes fitted at Madam Malkin's, Draco Malfoy told Harry that he wanted to go to Slytherin. Conversely, Neville Longbottom argued with the Sorting Hat for almost five minutes about being Sorted to Gryffindor; Neville didn't feel he was suited for Gryffindor and attempted to convince the Sorting Hat to send him to Hufflepuff. In this instance, the Sorting Hat chose not to grant a student placement in his desired house. We'll discuss this further when we formally take a look at the Sorting Hat itself and the phenomenon known as a 'Hatstall'.
² The way I read this, this is the moment the Sorting Hat detected the presence of Tom Riddle/Voldemort's soul inside of Harry.
³Emerson Spartz of Mugglenet - INTERVIEW WITH J.K. ROWLING - PART 2 OF 3 - 7.16.05
no subject
Date: 2013-01-28 03:12 pm (UTC)Harry definitely does have some "Slytherin" qualities. Remember, Dumbledore says to Harry that Harry has attributes that Salazar Slytherin valued; he does not say, "Harry, you are like Voldemort." I think that's a very important distinction, but the most important factor in Harry's Sorting was his choice:
Chamber of Secrets - chapter 18, Dobby's Reward
no subject
Date: 2013-02-03 10:24 am (UTC)This. Me too. ♥
no subject
Date: 2013-01-29 07:38 am (UTC)no subject
Date: 2013-01-31 08:40 pm (UTC)no subject
Date: 2013-02-03 10:28 am (UTC)