Dean's relationship with Castiel had never been particularly simple, or easy for that matter. They had been thrust together and expected to immediately get along, trust each other. Of course, Dean had disproved this with the amount of verbal abuse he had given Cas. Sometimes he felt bad because of all the sad looks he had seen in Cas' eyes, but the shock of the angel's appearance had rather distracted him from that.
They hadn't grown close quickly either. It had been a long and slow process, wha with Castiel's disappearing all over the place and Dean's lack of faith. But Cas had always wanted them to be close really, and, subconsciously, so had Dean.
They grew closer, spending more time with each other, helping each other. Frankly, after all the things they went through together, no one was really very surprised at how close they were. But no matter how close they seemed, there was always some distance. It was as if they still weren't telling each other everything. Everyone noticed, especially Sam, who was with Dean all the time. Dean himself noticed too, as did Castiel, but they both neglected to mention it for fear of how the other might react. However, it was always only a matter of time.
It wasn't exactly unexpected, therefore, when they did eventually bring it up.
Sam was out somewhere doing research. Dean had stayed in an Castiel had appeared announcing that he merely wished for some company from Dean. This was of course a little odd, but Dean said nothing. He liked Cas' company himself anyway. It seemed rare the angel stayed with him for any length of time.
Castiel then started to talk. He talked about Dean and himself and about the both of them. Dean listened, relieved they were finally addressing the issue. He joined in, saying about the strange distance between them. Castiel did not comment; he merely nodded. Dean found he could not stop himself talking, and so he continued to tell Castiel how he felt about the both of them until, to his horror, he found three little words slipping out of his mouth and he stopped. Castiel paused, then smiled and retuned the words in full.
There was such a moment of joy in both of them that they rather forgot themselves, so it wasn't much of a surprise to either of them to find that they were kissing each other.
It wasn't much of a surprise to Sam when he found them either.