To me, dialogue is the most natural part of writing. I can make people talk without even trying. If there is any purpose or structure, then lines will naturally flow from this task. And if there is no structure, then lines will flow from whatever the last line said was.
My friends have all seen at some point me speaking impromptu dialog between equally impromptu fictional characters. I change my voice and body position to show which character is speaking (though context generally gives it away). Again, this is a very natural thing for me to do.
I recommend that everybody starts talking with themselves. Become a character. Become two characters. Switch between them instantly and constantly. You never know what you might come up with.