'i could try to be big in the eyes of the world...'

this has been on my mind lately, and i don't think i even fully understand the significance of it.

in the hogwarts school of wizardry, where harry potter and his friends study the arcane, there are four different 'houses', named for their founders: gondric gryffindor, rowena ravenclaw, helga hufflepuff, and salazar slytherin.
when the school was founded centuries ago, the founders themselves chose which students would be in their house. the four wizards and witches would not be around forever and they created a 'sorting hat' and each placed a part of themselves in the hat, so that it would make the correct decisions. it is said that the hat is never wrong.

each house has its own characteristics and virtues.

gryffindor values chivalry, courage, and boldness. it is associated with the element of fire.

hufflepuff values hard work and patience, loyalty and friendship, and justice and fair play rather than a particular aptitude in its members. the house corresponds roughly with the element of earth.

ravenclaw prizes intelligence, knowledge, and wit. they are associated with the wind.

slytherin admires ambition and cunning. they are linked with the element of water.

gryffindor and slytherin are the two houses described in most detail in the books, as harry, his circle friends, their family members, and even dumbledore belong to gryffindor. nearly all the antagonists, from draco malfoy to lord voldemort, come from slytherin. rumors among the students are that all wizards who went bad came from slytherin, but that is not true, and there are many good wizards from that house as well.
cunning and ambition can lead to corruption and evil, but are not innately negative attributes.

the sorting hat holds a sort of premonition, seeing not only the student's immediate and obvious strengths, but latent and future attributes that may not have yet developed. some students in gryffindor don't immediately stand out as prime candidates, and hermione even asked the hat years after she was sorted why her intelligence didn't put her in ravenclaw. the hat replied that it had "considered it, but decided on Gryffindor in the end."

when harry was placed under the hat, it saw in him all the qualities to be great in slytherin, and weighed heavily putting him there. the house would help him achieve greatness, he was told. but harry did not want to go to house that rivalled his father and friends, the house from which voldemort began.
the students' personal preference is a determining factor in where they are placed, and that says a great deal about their character and desires.

when harry meets the younger persona tom riddle, who would rise to become lord voldemort and kill his parents, he sees many similarities between himself and the most evil wizard. this becomes quite unsettling, as he is afraid that he may be fated for something contrary to what he wants to be.

"Suddenly, something that was nagging at Harry came tumbling out of his mouth.
'Professor Dumbledore...Riddle said I'm like him. Strange likenesses, he said....'

'Did he, now?' said Dumbledore, looking thoughtfully at Harry from under his thick silver eyebrows. 'And what do you think, Harry?'

'I don't think I'm like him!' said Harry, more loudly than he'd intended. 'I mean, I'm--I'm in Gryffindor, I'm...'

But he fell silent, a lurking doubt resurfacing in his mind.

'Professor,' he start again after a moment. 'The Sorting Hat told me I'd--I'd have done well in Slytherin. Everyone thought I was Slytherin's heir for a while...because I can speak Parseltongue....'

'You can speak Parseltongue, Harry,' said Dumbledore calmly, 'because Lord Voldemort--who is the last remaining ancestor of Salazar Slytherin--can speak Parseltongue. Unless I'm mistaken, he transferred some of his own powers to you the night he gave you that scar. Not something he intended to do, I'm sure....'

'Voldemort put a bit of himself in me?' Harry said, thunderstruck.

'It certainly seems so.'

'So I should be in Slytherin,' Harry said, looking desperately into Dumbledore's face. 'The Sorting Hat could see Slytherin's power in me, and it--'

'Put you in Gryffindor,' said Dumbledore calmly. 'Listen to me, Harry. You happen to have many qualities Salazar Slytherin prized in his hand-picked students. His own very rare gift, Parseltongue--resourcefulness--determination--a certain disregard for rules,' he added, his mustache quivering again. 'Yet the Sorting Hat placed you in Gryffindor. You know why that was. Think.'

'It only put me in Gryffindor,' said Harry in a defeated voice, 'because I asked not to go in Slytherin....'

'Exactly,' said Dumbledore, beaming once more. 'Which makes you very different from Tom Riddle. It is our choices, Harry, that show what we truly are, far more than our abilities.'

(harry potter and the chamber of secrets, p. 332-333)