What makes a fanfic so good you'd wait months for the next chapter?

I got introduced to fanfic through friends and I guess it's something I keep going back to at different times in life. Some stories out there are honestly so good it's crazy how someone is doing it just for a hobby. But then there were also times I read stuff that was just a hot steaming pile of garbage.

So I guess what I am trying to figure out is what makes a fic actually worth going back to. Like what keeps you checking an author's profile hoping they finally posted a new chapter, even when their upload schedule is unpredictable as anything.

Is it the writing itself, the way they do the characters, the plot, the emotional gut punches? Or is it something you can't really put your finger on.

Just wanted to hear people's thoughts.

PS first time ever posting something on reddit, please go easy on me 🙏