Destiny. Some believe it, some believe in god given free will. They would be both wrong and perhaps headed in the right direction for it is not set in stone, our action will lead us to where we were meant to be but they can't make us take the plunge and the journey to the end, well.. There is where the loophole resides.
Certain things were set, Michael had to fight Lucifer, Sam would say Yes in Detroit and Dean would be the one to set it in motion, but that wasn't all there was to the story, it was all that they believed to be true, to matter.. but there was something else, a change that occurred on this journey which was the true miracle.
It wasn't that Sam said Yes, that Lucifer was free and not even that a demon decided to go against his creator, it was a man. A simple man that could change the course of it all but didn't, he was understanding and he was kind but there was no hope left in his mind and that, that made all the difference.
All of us are broken in some way, not many as excessively as Nick. He didn't have a easy life but nonetheless he grew up and met the love of his life, Sarah and for once, he felt content. Furthermore, nothing could bring him more joy than the birth of his daughter. Yes, they had their problems like any other family but they made it through, he knew he loved them more than life itself and they loved him.
Then one night he came home, not exactly sober but he knew what he saw wasn't an illusion. Their blood caked the house, a trail leading to his bedroom which he shared with Sarah, a pot previously full of boiling water was still dripping from where it had been knocked over. He found her first, it felt like a hit to the gut, his breath stolen and his mind took moments to comprehend the state her body was in. He was suspended in a moment in time for quite some time before the first thought broke through in an agonizing scream, what of his daughter.
Nick had ran to her room, her pink walls had stains which he could never forget, even now as years have passed he still sees them, his tiny doll was battered beyond recognition seemingly without reason and that, that was the thing that ultimately broke him. The realization that he finally lost everything he held dear and he couldn't fathom why.
Lucifer, however, was broken in a different way and yet the same, betrayal. It always came down to betrayal. His brother, his father, everyone watched as he was cast down by non other that Michael, who he had loved dearly. What made it worse was that he was right and no one would listen, they're still doing what they are told even after they witnessed how these creatures pillaged and plunged. He despised them, they were the reason ,he had believed.
He had sent Lilith back into Eden and still, they refused to see his point and for it he was not only banished but thrown into the cage to await the Apocalypse in which he had to fight his own brother, he couldn't fathom why but he felt like he had no choice but then after eons, the hate grew immensely and it was all he thought about, winning the apocalypse and killing every last one of these creatures.
The idea of free will and destiny is a complex one. Yes, Sam would say yes, yes Lucifer would fight Michael and yes, he did need a vessel but what drew Lucifer to this specific town, to this specific house holding this specific man. Was it fate or just coincidence that they shared a reminiscence of the same pain, it was never part of the plan, was it?
Them meeting, the chances were slim but Nick saying yes so easily even with the added torment was even more unlikely, but perhaps the most obscene was by pure coincidence they happen to share a pain that many could not fathom, not only that but it so happens that they had shared something else as well, a love that many could not begin to understand.
Nonetheless, it couldn't save either of them from what laid ahead for both of them, nothing could change the outcome for in the end, it was already set in stone.