In the context of this fanfic, Catherine Rollins and Steve are likely characters created by the fanfic author. They could be original characters, or they might be based on existing characters from a particular show, movie, or book. Without more context, it's hard to say exactly who they are other than two individuals whose relationship is central to the story where Catherine makes Steve cry.
One possible reason could be that she has lost interest in the particular fandom or the act of writing fanfic. Maybe she has new creative pursuits outside of the fanfic world, such as working on her original stories.
In this fanfic, Catherine realizes that she and Steve have grown apart. She sits him down one evening and says, 'Steve, we need to talk.' With a heavy heart, she tells him that their lives are going in different directions. Steve is shocked at first but then realizes she's right. Catherine leaves, and Steve is left alone, looking at the empty space she once occupied, feeling a mix of sadness and acceptance.
One possible plot could be about how her friends deal with the grief. They might gather to remember the good times they had with her, sharing stories and mementos.
One reason could be that their careers are taking them in different directions. Catherine might be getting opportunities in a different location that require her to be away from Steve for long periods, and they both realize that a long - distance relationship won't work for them.
One possible plot could be that her death sets off a chain of events that forces the Five - 0 team to re - examine past cases. Maybe she was involved in something that they didn't fully understand, and her death reveals hidden secrets. For example, she might have been in contact with an old enemy of the team, and that connection comes to light only after she dies.
One possible plot could be that Catherine Rollins reveals a long - held secret that Steve has been trying to keep buried. For example, she might expose a mistake he made in the past that cost his team dearly. This revelation shatters Steve's composure and makes him break down in tears as he realizes the consequences of his actions coming to light.
Her death could also have an impact on the villains in the fanfic. If she was involved in something that threatened them, they might feel a sense of relief at first. But then they would start to worry as the Five - 0 team closes in on them, suspecting that they were behind her death. This could lead to some interesting cat - and - mouse games between the villains and the team.