What Dean never knew was that Cas didn't have to hear them all the time. He chose to.
In fact, he only chose to hear Dean's every prayer. Dean was the only one whom he was willing to always listen to.
Also, he watched over the hunter every single night – some nights he felt lonely enough to make himself visible, so Dean might wake up and see him. Even if he only ever swore and called the angel a 'creep'; Cas was mad for every word that left Dean's mouth.
And, perhaps, if Dean had known this all along, things would have turned out differently.
Either way, at some point it was bound to happen. Everyone knew – Sam, Bobby, the entire garrison. Everyone except Dean and Cas.

As Cas' infatuation had grown he had increased his protection over the hunter, which resulted in him spending every free moment listening to what Dean was saying or invisibly watching what he was doing.
The angel knew that Dean wouldn't like it if he knew of this, but he truly had no control over his doing. He needed to know what Dean was doing; needed to know that he was all right.
It was pathetic: an angel of the lord so hopelessly chained to a human. At times, Castiel felt ashamed. Yet it never took him long to remember just why he loved Dean so – his righteousness, his virtue, his courage, his selflessness, … Yes, Dean was a better creature than any angel Castiel had ever encountered. And that was why he needed to be watched over diligently. After betraying heaven, it was the only thing that gave the angel a sense of purpose.
Dean was his purpose.

So he kept listening attentively until one day, he tuned in and heard an unfamiliar sound. Disbelieving, Cas flew over to the motel the Winchesters were currently staying in, and he found Dean lying on his bed – Sam was nowhere to be seen or sensed – his head buried in one of the pillows and the sound—Cas could hardly believe it—was, in fact, sobbing.
He was glad he'd chosen to make himself invisible to human eyes, for he knew that Dean would be horribly embarrassed if he knew that someone saw him cry.
But why was he crying? Cas did not know.
He simply stood a few feet away, silently watching Dean's shoulders tremble and his arms tighten around the pillow.
It broke Castiel's heart to see him like this.

Yet when he wouldn't stop crying, when the sobbing grew more and more intense and desperate, Castiel shrugged off the cloak of invisibility and silently stepped closer to the bed. "Dean." He said.
Dean jumped, sitting up and drying the tears with his sleeve instantly. "God damn it, Cas!" He cursed. "Can't you grant a guy a bit of privacy once in a frickin' while?!" He seemed to be genuinely angry, but Castiel saw through the anger and recognised the sorrow that still lay within Dean's eyes and mind.
"Dean, please don't cry." The angel said demurely. He didn't want to upset Dean even more by being present but at the same time he couldn't leave him alone with his grief either. It was a difficult situation to be in and, frankly, all Cas wanted to do was wrap his vessel's arms around Dean's frail human body and rock him back and forth like he had seen it be done by mothers and lovers.
Dean chuckled bitterly. "Right." He said – all red swollen eyes and wet cheeks, and Cas still found him to be the most beautiful thing he had ever seen. "Thanks for the advice, Cas." He kept rubbing his eyes as though he thought that the rubbing would prevent the tears from welling in them. But it didn't. The tears kept coming and Dean still looked angry and Castiel took a cautious step forward.
"Fuck off, Cas." Dean said, his voice turning into a sob as he said the angel's name.
"Dean, I don't mean to invade your privacy but…" Cas hesitated and hesitantly stepped even closer. "I would like to hold you."
The angel was very confused at the sound Dean made in reply to his request. It was a mixture of laughter and sobbing and sounded very odd.
"I told you to fuck off." Dean managed to gasp, drawing away from Cas who was inching closer.
"Dean, I think I can do it. I've been having very human feelings for you… I would like to at least try. It's supposed to be very comforting." Saying this, Castiel sat down on the bed beside the hunter, watching him intently.
"You're crazy." Dean said, still crying.
"Please." Cas said, aching to help his beloved human.
"Fuck off." Dean repeated but the sobs took over his voice again and Cas could almost hear the sound of his resistance crumbling.
He turned to face the hunter and carefully, slowly and tenderly he put his arms around Dean, who pulled away violently for a split second before sinking into Castiel's chest, tears streaming down his face.
Cas found that hugging wasn't as difficult as it looked. He just did what he'd wanted to do since he'd first laid his hand upon Dean in hell. He held him close, closer than anyone else would ever get, and cradled him gently.
Dean sobbed wordlessly and after a while almost silently. Eventually the tears stopped and all that was left was heavy breathing – which sounded suspiciously like hyperventilating – and trembling.
"Cas." Dean croaked, head still resting against the angel's chest. "Why can't I be happy?"
Castiel thought about it for a few moments then he leaned down to lay his cheek upon Dean's soft hair, quietly answering, "Because you're too good, Dean.
"I don't know why, but it's always the good ones who must suffer."
Dean's chuckle was wry and hoarse. "I'm the worst… the worst brother, the worst son, the worst friend, the worst human being… I'm… I'm disgusting." And as he said it a new flood of tears clouded his eyes.
Only then did Cas realise that the man before him was seriously inebriated. Yet it didn't matter.
Castiel pulled away a little to reach up and hold Dean's face in his hands, only a few inches away from his own. His palm brushed against surprisingly soft cheek and Cas smiled a small, fond smile.
"You're the best." He said in a steady and strong voice. "The best hunter, brother and friend. You were the best son. You're…" All of the sudden Cas felt coy, and he stammered for a short moment. "You're… the best thing that has ever happened to me.
"I just wish you could see." He added in a soft, sad voice.
Dean stared at him. Stared and stared and stared until Cas felt genuinely uncomfortable. Then Dean leant in and kissed him. Just like that – pushed his lips against the angel's and let them linger.
Castiel's eyes fluttered closed and open again, not sure what to do except relish the feeling. It was the sweetest feeling he had ever known.
He was still palming both of Dean's cheeks and when the hunter pulled away a little Cas let his fingers caress the delicate lines of his face.
Dean looked awestruck and incredulous, so Cas leant forward again to brush his lips against Dean's in another sweet kiss.
He had never kissed like this before. He'd never loved like this before.
Neither of them had ever loved like this before.