A common aspect in such fanfiction could be the build - up of emotions. Dean and Castiel have a complex relationship throughout the series. Their first kiss might be a culmination of unspoken feelings, perhaps after a dangerous situation where they realize their true feelings for each other. It could also be a moment of great vulnerability for both characters, as they break through the barriers of their friendship or comradeship.
You can try searching on fanfiction websites like Archive of Our Own (AO3). There are a large number of Supernatural - related fanfictions there, and you're likely to find ones with Castiel and Dean having a kiss. Another option could be FanFiction.net, although the content and quality may vary more widely.
One popular trope is the 'First Kiss' trope. It often builds up a lot of tension between Castiel and Dean before they finally share that first, passionate kiss. Another is the 'Forbidden Love' trope, as their relationship within the context of the show has many obstacles, and this is often explored in fanfiction with their kisses being something that goes against the norm or is kept secret.
One possible theme is the exploration of forbidden love. Given their different natures (Dean being human and Castiel being an angel), their relationship might be seen as something that goes against the natural order in the context of the show's universe. The first kiss then becomes a bold statement against those restrictions.
You can start by looking on Archive of Our Own (AO3). It has a vast collection of Supernatural - related fanfiction, including many stories about Dean and Castiel's relationship that might feature a kiss. Just use the search function and type in relevant tags like 'Dean x Castiel' and 'kiss'.
Well, 'The Unspoken Love' is also quite well - known. It's a slow - burn story where the kiss is the culmination of many small, tender moments between Dean and Castiel. It shows how their relationship has been building gradually, like a slow - growing flame.
Often, the theme of sacrifice is present. Maybe one of them has to make a big sacrifice for the other, and the kiss is a moment of intense emotion before or after that. It could also be about them breaking free from some kind of supernatural constraint that has been holding them back from being together, and the kiss is a symbol of their newfound freedom.
In dean - castiel relationship in such fanfiction, there's often a deep emotional connection. They might show a side of extreme vulnerability towards each other. Their relationship can be intense, with a lot of passion and a sense of 'destined to be together' kind of feeling.
You can start by looking on Archive of Our Own (AO3). It has a vast collection of Supernatural fanfiction, including many that focus on the relationship between Dean and Castiel with kissing scenes. Another option is Fanfiction.net, though the quality can vary more there.
Another excellent fanfiction is 'Heaven's Gatekeepers'. This one plays with the idea of their roles in the grand scheme of things in the Supernatural universe. It has a great plot that weaves in and out of canon events, and the way it portrays Dean and Castiel's connection is both unique and captivating.