Set around the time of The man who would be king, Season 6 episode 20
If Castiel was anything, it was a coward. A coward so low, he could not even face his friends, his family. He could not even face himself. And so, he turned to God. His god, whom he loved. Whom he believed in. Whom he trusted. He turned to God for guidance, and God turned away. The sad story falling on deaf ears, from the mouth of Castiel the coward. He betrayed them. He felt guilt for that. He knew what they thought. Knew they no longer trusted him. And he knew they thought this. He listened to them. Watching them when they thought he was gone. And yet, no matter how they begged, no matter how bad it sounded, Castiel could not bring himself to regret it. It was not right. Not moral. Just tactical. Logical. If only they could see it his way. To lessen the guilt that was slowly consuming him, corroding his resolve. So he watched them. He watched them but they did not see. Yet another betrayal. Another invasion of privacy. He watched them anyway. It was one of the few things he could draw comfort from. The thought that it all was for them. To protect them, keep them safe. To keep Dean safe.
Castiel couldn't deny his growing affection for the stubborn human. True, he loved Sam and Bobby, just as he loved all god's creations, maybe even a bit more. But this, this was different. Castiel found himself watching Dean more and more. Cataloguing his movements, memorizing each expression, each pout of those perfect lips. Those lips. Castiel found he could stare at them for hours on end. Just stare. He never touched, though he often found himself shuffling forward, hand outstretched, as if to trace that perfect jaw line, or even just to hold him. Dean looked like he needed to be held a lot, and it was all Castiel could do to stop himself from gathering Dean up into his arms and clinging to him. Cling to him till Dean's hands stopped shaking, till he finally relaxed, till he felt safe.
And it was a day like this one that Castiel found himself closer than he had dared get before. Even invisible, Castiel was cautious of Deans form. It had never bothered him before, being near him, though Dean had brought it to his attention numerous times, using words like "Personal Space". Now Castiel was scared to get too close. He felt to do so was to add to the growing list of betrayals towards the man Castiel now realized he loved.
And yet, here Cas was, mere inches away from Deans face. So close. Close enough to touch. To lean forward just a little, and brush his lips against Deans. And he did. Ever so slightly. Just a small brush. A shared breath.
Dean leapt backward like he had been kicked, his eyes scanning the room wildly, searching for the source of the touch. He eyes passed blankly over the space where Castiel was, invisible to the human eye. Castiel fled, panicked and confused. How could he? He invaded Deans space, touching him without permission. He felt sick. Conflicted. He wanted to. He had wanted to kiss Dean so bad it had hurt. But Castiel had hurt before, and he had suffered. He had no excuse. He felt like crying, and if he had been anyone else, he may have. But he was Castiel, angel of the lord, and he would not cry. He had betrayed Dean. He did not deserve to be happy. He still didn't regret what he was doing. He would do anything to keep Dean safe, even if that meant opening the gates to purgatory. And it did.