Because the actual TG manga is waaaaaaay tooooo sad and dramatic?
Amon Koutaru, your father, is one of the best ghoul investigators of Tokyo. Your mother died when you were 2 years old, due an unhealable disease, that was what your father always told you when you asked him about your mother. One day, when it was already dark outside, you decided to spend some time with your friends and went to the cinema, since your father wasn't at home because of his work, you basically could do whatever you wanted. But you couldn't expect what you would see that night...
This is much rather a mysterious fanfiction than a romantic one, but oh well.