I constantly think about the fact that sam was the good one in the family, the more empathetic and moral one, yet he was the devil’s vessel; while dean, who was more explosive and impetuous, was michael’s sword. all this while sam had a demon by his side, and dean, an angel. however, even though sam was hell’s prince or whatever, he still needed something to actually corrupt him (ruby, the demon blood), while even though dean was the righteous man, he was the one that ultimately was doing the corruption (made castiel fall and made him rebel against heaven and God Himself). “The very touch of you corrupts. When Castiel first laid a hand on you in hell, he was lost.” this plus dean being dad’s perfect soldier and sam the favorite rebellious son, which sealed their destiny: it could’ve only been them in the end, the perfect lucifer and michael.