"You might belong in Gryffindor,
Where dwell the brave at heart,
Their daring, nerve and chivalry
Set Gryffindors apart"
—The Sorting Hat
There are no bad eggs in the Gryffindor House, this is how it works. The good people go to the lion, the raven takes the cleverest of the bunch, the snake takes those who are rotten and the badger takes the rest. Everyone knows this, this is how it works. There are no bad Gryffindors, like there are no stupid Ravenclaws, cruel Hufflepuffs or good Slytherins. everyone knows this, this is how it works.
No one goes against the status quo. This is how is goes.
They call James Potter a perfect Gryffindor. He is brave and strong and handsome--he even got the beautiful girl with hair so red it looked like a cascade of blood. James Potter is good with a wand and his smile is quick and fast, with glasses glint gold. James Potter is a pure-blood, but doesn't care about your blood (unless, of course, if you're a filthy Slytherin and then he will disdain you, hex you, taunt you, even if you are a clueless first year who knows nothing of politics).
They call Sirius Black an adequate Gryffindor. He comes from questionable ilk (House Black rots but its purity--well, Tojours Pur) but like his best friend, he is brave and strong and handsome--he doesn't have a girl, as such, more like girls. No one minds because he's Sirius Black and probably the most popular boy at Hogwarts. Sirius Black is talented and will always have your back. He doesn't care about your blood status (unless, of course, he associates you with this family and then you better and run and hide from his sneer and quick curse when McGonagall isn't looking).
They call Remus Lupin a mediocre Gryffindor. He's always ill, not particularly brave or strong or handsome like his friends, and he is so clever that many think he would be better suited with Rowena's students than Godric's. But everyone has seen his pranks and his scheming and he always holds doors open, there's bravery and chivalry there, just well hidden, and he has a lion's heart. Remus Lupin is a half-blood and honestly doesn't care what you think about him or care about you, really (apathy has a name and it's name is Remus).
They call Petter Pettigrew--well, alot.
They tell you that Gryffindor is the home of the brave, the honourable. Those with courage in their hearts and chivalry in their souls--a Gryffindor will never back down, they say, but they say it fondly, as if the inability walk away from a fight, step away from a cause is some sort of positive character trait. They are valiant soldiers who will fight and die for the commander, the old man who sees nothing of the front line.
(albus dumbledore is the greatest wizard who has ever lived, you are told again and again and again and again and again and you can longer tell the difference between truth and lie, myth and memory)
You see the lie first. You notice that not only are we loosing, but we are loosing badly. For every Death Eater taken, ten Order members fall. Dark Mark's are so common-place now many barely bother hiding them and attacks are so regular people don't bother to waste their breath on screaming when the black plumes of smoke fall from the sky and snatch another unfortunate soul.
The lie is a simple one. Like good little Gryffindor boys they had always believed that good would always conquer evil, the lion will always kill the snake. They bury Marlene McKinnion, Fabian and Gideon, Benjey Fenwick. Caradoc disappears in the middle of the night. Bellatrix Black attends Gala's within the heart of the Ministry of Magic, her husband lazily draped around her arm. Narcissa Black becomes Narcissa Malfoy and her marriage is on the cover of the Daily Prophet. You watch as evil prospers and good is stamped out, like some persistent weed, and cast aside everything you thought you knew. What is being brave at heart when your entire family is murdered? In a society ruled by a racist elite, what does it matter about being daring?
You watch as it all implodes and you make a choice.
You just want it all to end.
(it's an added bonus that it would end and you'd be on the winning side)
You always knew you'd end the war, one way or another.