"What's the harm?"
There's a reason why most people on the tube are normally either deep in a book or the newspaper, playing with their phone, or listening to something on headphones. It's to get through the journey as quickly as possible, while disturbing as few people as possible. And two (or more) people having a "chat" is not a chat - it's a public conversation with everyone in the immediate vicinity being forced to listen to it, whether or not they want to.
God knows the tube is stressful enough at rush hour, without people "chatting" all around us.