I also know a monomaniac. When we were young, it didn't seem so bad. She was funny and full of energy and we laughed a lot. But as she's aged, the monomania has become unbearable. She just doesn't listen to a thing I say. Often, she will cut across me, to shut me up. She also has a loud voice, which she uses to silence people (I wonder if it's common for narcissists/egomaniacs/monomaniacs, etc, to develop loud voices?). I can see her literally glaze over as I'm speaking. Like a lot of narcs, however, she's good at manipulating people. Self-centred people need others. They need and crave an audience. So they will pretend to care in order to get that audience. I've watched her do it – faking an interest in other people to get them down the pub, where she can then turn the conversation onto herself. I've gradually realized that what she really loves isn't so much talking about herself as listening to herself talk. Her idea of heaven would be a bar with a big mirror where she could watch herself talking to an audience. Every now and then someone would ask her a question (but they'd be quick, because it literally causes her pain, I mean actual physical pain, to be silent while someone speaks)
She also talks about her children all the time. Her kids have become an extension of her ego, and their achievements are her achievements. I'm sick to death of hearing how hilarious the young one is, how she has the class at school "in stitches," how clever and academic the older one is, and so on. As others have said, no one is interested in other people's children.
On top of all that, she's boring generally. She's very bright and articulate, but she's never read a book and, even though she could talk for five hours without a pause, isn't actually interested in anything. It's weird to be with someone who talks non-stop yet isn't actually interested in things. I mean she has no interest in, say, books, or science, or the future, or anything like that. It's like her purpose in life is to make herself the centre of attention, boost her ego, dominate/beat others and enjoy listening to herself talk.
Truly interesting people are interested. They are interested in art or literature or travel or whatever. They are also interested in other people (not in a nosy, spiteful, gossipy way; I mean they want to hear what you think about a new art exhibition, or the World Cup or Prince Harry, etc etc).
Phew, sorry...enjoyed that rant. I know someone will ask why I don't just ditch this person. It isn't that simple. Cutting old friends out of your life is hard. We live near to one another, have mutual friends, and so on.