Well, it could be a really sweet moment. Maybe it happens after they've been through a tough case together. They've been growing closer and closer, and one day, after sharing a deep conversation, their eyes meet and they just can't help but kiss. It would be a moment full of unspoken feelings finally being expressed.
Perhaps in the fanfiction, they are training together for a new police technique. They keep messing up and laughing at each other. But then, in a moment of stillness, they realize how much they enjoy each other's company, and the first kiss is a gentle, spontaneous expression of that new - found feeling.
Yes. The kiss between Natsu and Lucy is a pivotal moment that adds depth to their relationship in the manga. It comes as a surprise to many but is a welcome development for their love story.