People keep asking this question on the Sophie Fitz ship page, but I avoid a long explaination because I know it will turn out to be a ship war. Anyway, I'm going to answer with what I currently remember, but I'll add more later. I'm not going to go into moments, because there are waaaay to many of those.
- It just feels right. (Basically, my heart tells me to.)
- They're adorable.
- Sophie has had a crush on him since book one.
- They're cognates (in training).
- Fitz is the person who brought her to the Lost Cities.
- Fitz and Sophie both trust each other immensely.
- Fitz is the one other Telepath (Other than Mr. Forkle) who can enter Sophie's mind.
- Fitz can boost Sophie Telepathially.
- And so on!