What causes a person with dementia to talk endlessly for hours?

It’s called verbal agitation and perhaps it happens because the person has lost some inhibitions that remind him to stop talking. I also think that sometimes they are saying, “I’m See also:
Non-Alzheimer's Causes of Dementia still here” and the endless talking or physical movements are ways they can communicate that. This behavior passes as the dementia eventually affects the ability to speak.  If this behavior bothers him, medication to calm him may help. If it bothers you, but not him I’m afraid you must put up with it. 

If he can eat something or safely suck on a lollypop it might keep him quiet for some period of time. You can also try headphones with music he might enjoy. 

Dear Joyce,

Thank you so much for addressing this issue, and Rendezvous 747, hang in there... We have listened to our loved one with dementia incessantly, trying to make eye contact and responding as much as we can, and sometimes ignoring for a while. However, during the past few days things became intolerable since she accuses everybody of killing someone else! We try to be sympathetic, humorous, we tried to show that the person she claims is dead is alive and well. Suddenly she wants me out of the house for trying to rationalize or respond to these comments. Distractions won't work either. We are afraid that this is a sign of further deterioration on her condition. Anyways thanks again for sharing your thoughts and advice. Blueberry