One thing that is tricky about dementia is that people literally have trouble remembering all the times that they forget things. It is for this reason that they often underestimate how impaired their memory is—or they may not remember that they are impaired at all. Similarly, as their hippocampus deteriorates, they may completely forget that they have dementia, such that every time they are told they have the disorder, they experience the news as if they are hearing it for the first time.
From Psychology Today
People often forget that they have dementia
One thing that is tricky about dementia is that people literally have trouble remembering all the times that they forget things. It is for this reason that they often underestimate how impaired their memory is—or they may not remember that they are impaired at all. Similarly, as their hippocampus deteriorates, they may completely forget that they have dementia, such that every time they are told they have the disorder, they experience the news as if they are hearing it for the first time.