I'm not sure specifically as I haven't read this exact novel. But generally, it might be about unrequited love, where the protagonist has to deal with the situation of the other person not loving them back.
The plot of 'Don't Say You Love Me' manga is centered around the lives of a group of individuals who struggle with expressing and understanding love. There are misunderstandings, unexpected twists, and moments of self-discovery.
Since I have no information about this novel specifically, it's likely that there is a protagonist who experiences this unrequited love. There might also be friends or family members of the protagonist who play important roles in either supporting or challenging their feelings. There could be a rival in love as well, if the story takes that turn. Without reading the novel, it's all speculation, though.
Well, it could be that the manga has deep character development or explores themes in a thought-provoking way that makes blaming it unjustified. Maybe the issues people might have with it are more about personal preferences than actual flaws in the work itself.