Gu Zhou was not born because his parents liked him. After he was born, he became a medicinal source. His world was dark.
Perhaps Gu Zhou had been unwilling to tell her Gu Qi's identity in the past because he wanted to protect him. He did not believe that there was anyone else in this world who could protect Gu Qi.
He probably didn't want Gu Qi to experience his previous pain and darkness again.
Gu Zhou might also think that she was just trying to get rid of her bad luck by marrying him. He might not think there was a need to tell her this.
Or perhaps, Gu Zhou did not want to argue with her.
If she were Gu Zhou, she might not be willing to say it either.
