You were an angel whom escaped Heaven, like Anna, and left your grace. You were reborn as a Winchester but no one besides Castiel knows that. He needs to watch over you since Sam said so, saying you were in danger by Lilith

"You fled from me, fell to live amoung the humans and purposely kept yourself hidden- thought to be dead- in hopes I would not find you" Castiel glares at her, his voice demanding and slightly harsh "There is no outcome tonight in which I will be leaving you alone"