It was the Mark. The lock. Whatever it was he had used to lock away the Darkness. Gabriel had seen it eat away at his brother until all of them were fighting, at each other's throats. It shouldn't have taken Him by surprise, but it did.

Soulmates. A human bonded to another and an angel. An archangel. Slowly it dawns on Gabriel and he drops his head into his hand, swearing softly. "Dad. Damnit." That's why he'd felt the sudden connection with Sam the first time they'd met, why nothing felt complete or whole unless he was with the two humans he loved most in the world.

Why he didn't hesitate to sacrifice himself to protect Sam.

"That's why we could call Sam back from the edge."