To Castiel, Dean's prayers were always louder than everyone else's.
That was because Castiel was Dean's angel.
Each angel, when created, is assigned a human to look over - basically as a guardian angel.
Normally, the human would never get to meet their angel, never even know they had one. Dean was one of the first.
Technically, Sam was the first ever human to meet his guardian angel - Gabriel had been assigned to look over the younger Winchester - but because the brothers didn't know what the Trickster truly was, Dean was the first to know about his guardian angel. Sort of.
—
When a human met their guardian angel, they would form a bond. Soul-to-grace, so the pair would forever be connected.
It would mean the angel always felt a pull to their charge. You couldn't just leave your human alone, especially after revealing yourself to them.
—
Guardian angels' always heard their charge's prayers louder than any other. That's why Cas didn't hear a lot of Sam's prayers. Longing and thoughts sometimes came across as prayers - especially if they were about the angel - and so Dean drowned out Sam most of the time.
He knew that he could never explain the reason why Cas always replied straight away to every one of Dean's prayers, but very rarely to Sam's, because he wasn't sure how it would go down with the brothers.
Especially because Dean's longing and thoughts occasionally bordered inappropriate, and the socially-inept Castiel wasn't quite sure how to handle what would be the result of that conversation.
—
Usually, the prayers and thoughts would go quiet when Cas was in the same room as Dean. But, on one particularly normal afternoon, spent lounging around in the bunker, it was as though Dean's prayers were so loud, they sounded in Cas' head as a whisper, even while he was looking straight into the man's eyes.
'I really want to kiss him.' was the thought that rang the loudest, and Castiel, thrown by the new situation, threw caution to the wind, and answered Dean's prayers the best way he could.
As soon as his lips were on his charge's, thoughts went slightly inappropriate, but the guardian angel was going to answer each and every one of his human's prayers.
Even if he prayed for the same thing a thousand times over.