One popular trope is the 'hurt/comfort' scenario. For example, Dean gets injured on a hunt, and Cas takes care of him, which leads to a tender moment and a kiss. It plays on the emotional bond between them and the protectiveness they feel for each other.
The 'soulmate' trope is also quite common. In these stories, Cas and Dean are fated to be together, and the discovery of their soulmate connection often leads to a passionate kiss. It adds a sense of destiny to their relationship.
I really like 'The Kiss in the Rain'. It's a bit of a classic in the dean and cas fanfiction world. The rain - soaked setting for their kiss adds a lot of drama and emotion. The story around it also does a good job of showing how their relationship has evolved over time.
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 common theme is the struggle with their identities. Both Dean and Cas have complex identities and in many fanfictions, their kiss is a moment of acceptance of themselves and each other. For example, Cas coming to terms with his feelings as an angel while having human - like emotions for Dean.
One popular theme is the friendship - to - more relationship. Fans often explore how their deep bond could develop into something romantic. Another theme is their battles together against the supernatural. Writers love to describe their teamwork and how they rely on each other in dangerous situations.
One common theme is the revelation of hidden feelings. For a long time, they may have been suppressing their emotions for each other, and the first kiss is like the breaking of a dam. Another theme is the acceptance of a new aspect of their relationship. It's not just about being friends or hunting partners anymore. And finally, there's often a sense of new beginnings, as if this kiss opens up a whole new world of possibilities for their relationship.
Well, in many Cas and Dean first kiss fanfictions, it often occurs in a moment of high emotion. Maybe after they've just saved each other from a dangerous situation. The writers usually build up a lot of tension between them first. For example, there might be lingering looks and unspoken feelings for a long time before that first kiss. It's a way to explore their relationship deeper than what's shown in the original series.
One popular trope is the 'Soulmate' trope. It adds a deep emotional connection between Dean and Cas, which can make the lemon aspect more intense and meaningful. For example, they might discover they are soulmates through some magical or supernatural means and then their physical relationship develops from that deep bond.
The idea of 'forbidden love' is also common. Maybe because of their different natures (Dean being a hunter and Cas being an angel), or because of some external forces like the rules of their respective groups. This adds an extra layer of drama to the story. And of course, there's the theme of pure, unadulterated love where the kiss is just a beautiful expression of their deep feelings for each other.
The main reason is the relationship between Cas and Dean. Their dynamic in the show is complex and full of unspoken feelings. Fanfiction allows fans to explore that further. For example, they can write about what could have happened if certain events in the show turned out differently.
You can try some popular fanfiction websites like Archive of Our Own (AO3). It has a vast collection of Supernatural fanfictions, and you're likely to find many stories about Cas and Dean with their romantic moments. Just use the search function and type in relevant keywords.