A true simp story could be about a guy who always buys his crush expensive gifts without expecting anything in return. He just does it out of pure adoration.
It could be that their storylines and personalities offer an escape or a form of entertainment that resonates with you. Sometimes, we simp for them because they represent something we wish we had or could be.
A simple Nezuko and Zenitsu fanfiction could be that they go on a journey together. Along the way, they encounter other demon slayers who are initially skeptical of Nezuko. But Zenitsu stands up for her, explaining that she's different. This shows the depth of their friendship and how Zenitsu has come to trust Nezuko completely.