"You bleed. I bleed. They bleed. Whether we're Muggle-born or pure-blood or half-blood, it's just blood. What does it matter where our blood comes from when it runs through our bodies the same?" "It's the quality. It's not about whether we bleed or not. It's not about the purpose. It's about the quality. And the quality of our blood is different: it's better." SiriusOC

Because respectable Slytherins of pure-blood don't fall in love with obnoxious blood-traitors in Gryffindor. [SiriusOC]