One possible theme could be the consequences of human actions. Since it involves 'willful negligence', it might explore how small acts of neglect can have big impacts on love and relationships in a historical context.
I'm not sure specifically which novel 'love history caused by willful negligence novel' is. But some general romantic historical novels like 'Pride and Prejudice' could be somewhat similar in terms of exploring relationships within a certain historical context.
The main theme might be about how wilful negligence can lead to the development of a love story. It could explore how certain inadvertent actions or lack of attention result in unexpected romantic connections.
The 'unintentional' aspect makes it special. In many love stories, the characters actively pursue love, but here it might be more of a gradual discovery of feelings that they didn't initially expect. This gives a more natural and unexpected feel to the relationship development.
History BL novels often have rich historical settings. For example, they might be set in ancient Rome, medieval Europe or feudal Japan. This setting gives a unique backdrop to the love story.
They usually feature detailed world - building. For example, the architecture, clothing styles, and social systems of the historical period are well - described, which helps to create an immersive reading experience. Also, the emotional development between the male - male pairs is often slow - burning and intense, filled with longing, misunderstandings, and finally, deep love.