You, Dean, and Sam have been hunting together ever since their dad had gone missing, and Jessica passed. You knew both men when they were just young boys, and you were a young girl since their dad was a close hunting partner to your dad.
Over the years, you had fallen absolutely in love with Sam, and vice versa.
Sam had become addicted to demon blood and used it to stop Lilith whilst Ruby tricked him into it. The night before Sam went to kill Lilith, you had confessed you were in love with him. He had taken this to heart and kept it with him as you let him go to hunt down Lilith with Ruby. You put your trust in Sam and asked one thing if him. 'Don't let me down.'
But, things went little did Sam know that when Lilith was the last seal, and when he killed her, that released Lucifer from his cage. Sam was overcome with guilt and shame. And Dean? Well, Dean was absolutely livid and could barely look at Sam.
Sam was afraid you'd feel the same way as Dean. That you'd think he was a monster. That you'd berate him and change your mind. That you'd stop loving him. So, he kept his distance so he didn't have to deal with the heartache of never getting a chance with you.
Until... one night, Dean was out, leaving you and Sam alone in the motel room...