Years ago, a friend of mine was teaching college English when she noted that one student wrote every essay on birds.
She suggested he write about something else. He said he couldn’t. “What,” he asked, “are you going to do with an idea that won’t let you go?”
The French have a name for this condition. It’s called an idee fixe. I knew a man who loved to talk about Cokes so much that during every visit he would get 15 minutes to declaim about Cokes: cherry Cokes, diet Cokes, new and old Cokes; cokes versus Pepsi.