"Nick has always been a little inadequate," he concedes, because it's a true enough observation. "He isn't my true vessel. He was always just a stop-gap until I could get to the right one."

Sam wouldn't be a prison-- Sam was his, Sam was born to house him. Lucifer's name is written in the very fibers of Sam Winchester's being, on sinew and bone and soul, because their union is literally the Word of God. And what God has brought together, let no power pull asunder.

"Angels can't manifest on Earth for very long without a vessel. Our presence is too overwhelming for mortals' fragile forms. Just the sight of us would burn their eyes from their skulls and our voices would shatter their minds and well as their ear drums. Humans aren't equipped to deal with the full glory of my brethren, so..." he plucks at his shirt, "...hence, the vessels."