Castiel was introduced into the story in season 4 because they needed a way for Dean to escape Hell, and then they kept the character when he and Misha proved popular. When the ship of Destiel became popular, the writers began teasing it in their writing.

I do think some fans did jump on the idea of Destiel as the first non-related relationship between main characters, though.

@Flickerflame That's not it. Destiel was introduced because some moral puritans in the company which made SPN worried too much just how bad would it look like if they had 2 brothers who live in incest. To please the bosses, Kripke simply invented some bloody angel, and put him in the show, right between Sam and Dean. That ruined Wincest forever. Dean fell in love with Cas the moment he saw him, and he forgot all about his brother and his lover, Sam. I fucking hate when it happens! There should be just 2 brothers who are also lovers at the same time. There was no need for any angels in the story, because this show was all about demons, not angels. It always was. There was no angels before season 4, and if you ask me, there never shoud be any angel at all. I hate fucking angels. Cas ruined the entire Wincest fandom by himself, and it's all Kripke's fault!

^The angels were always planned to be part of the story. Kripke had the overall structure of the first five seasons planned when he started writing the series. Castiel was kept around instead of being killed off at the end of s4 because of how popular he'd proven to be, but the angels, the apocalypse and the idea of both brothers being vessels was always planned.