When Kakashi was a child, he had a broken home, overwhelming ninjutsu scrolls, and the lonely world of a genius child. In fact, the same goes for the Fourth Generation, Shisui and Itachi.
However, Kakashi later had four generations of teachings. Itachi had Shisui taking care of him.
For the fourth generation, Kakashi is his teacher, father, brother, only relative, only support, and only attachment. To Itachi, Shisui was his older brother, his friend, the only one who didn't treat Itachi as a genius and restrained him, the only one who understood Itachi, and the only one in the entire family that Itachi liked and was attached to.
The fourth generation taught students to protect Konoha. Zhisui said to Itachi, I want to see your future.
Therefore, the fourth generation died, and Kakashi closed himself. Shisui died, Itachi exterminated the clan and defected.
Conception: