Lost
Ah, in the end you can give someone all you have to give and it still won’t be enough. Being dutiful, responsible, loyal— it can’t make people love you if what you are is not lovable. That has to be the reason. He is not loveable. There is nothing in him for someone can love.
It is an issue of personality mainly, he thinks. He is not sweet, he is not funny, he is not flippant, he is not light-hearted, he is not generous, he is bitter, and spiteful, and jealous, and intense, and cruel, and unattractively needy.
When the aftermath of the events of canon occurs in a world in which Jiang Cheng and Lan Wangji had a longstanding, undefined, and not entirely happy sexual relationship since Jiang Cheng went to study in the Cloud Recesses, a relationship that ended as Wei Wuxian rose again.
Note: Chapter one of this is largely my fic "The Death of Hope" which I have reworked and added to. From then on it will be new content.