The drive had been long and stressful. All the Winchester boys could think about was the memories and time they lost. All the things that could have been, but were sadly not meant to be. Life would have been so very different had Mary Winchester not walked into Sam's room all those years ago.
The same could also be said that everything would be different had she not made a deal with Azazel, or if John hadn't died in the first place. So many could be's, would be's, should've been's. There was no point in dwelling over lost time.
That's didn't change much though, Sam still thought about the life he could have had. A loving family with an ass of a brother, Stanford, Jess. A life outside of this. Dean dwelled on the few memories he had of Mary. Getting the crust cut from his sandwiches, hearing Mary sing Hey Jude when he was upset, or hearing her tell him angels were watching over him. She never knew how right she was.
Castiel was sitting shotgun with Dean, holding onto his hand for a small form of support as the other man drive. Sam wouldn't have been much help; he was just as mopey as Dean was. Castiel, on the other hand, didn't know Mary. He didn't have to picture a life with her in it, or so Dean thought.
On this drive along the twisting back roads, run down motels and roadside diners Castiel daydreamed. He thought up a life where he was human, simply and utterly mortal. No fear of his family, no secrets or guilt, just the easy life of a regular man. Maybe if he had been human he'd be some sort of musician, music was one of Castiel's greater loves of humanity.
In this little dream Dean would bring him home to meet his parents. Castiel would be nervous of meeting John, hearing many contradicting things about him, but meeting Mary would be the highlight of his day. A mother so loving and nurturing that all she wanted for her children was for them to be happy. It didn't matter what they did for a living, who they loved, just as long as they loved the lives they were leading.
She'd welcome Castiel into her home, hug him gently but warmly, as only a mother, or someone just as dear as one, could.
Sadly, none of this would ever happen.
No trips home, no tender smiles or caring hugs. There would be no Thanksgiving or Christmas dinner, no grandchildren for Mary and John to meet. They'd never meet their children's own, loved, bouncy bundles of energy. They were just ghosts, after all. Their faces were gone, their names simple memories. The touch they left behind has surely faded.
They were the specters hiding in the corners of the Winchester mind, rarely showing themselves.
Gone they were, and there was nothing to be done about it. Not even God could bring them back.
Even if he could, why in the world would he?