One of the reasons is that you aren't occupying your brain on when to pause incase the other person wants to chime in, how the other person is reacting, what to say if he reacts in a positive/negative way, etc. That's why it's so important to practise and expand your horizons from talking to yourself, talking to your family members, talking to your close group of friends, talking to all your classmates, talking to the shopkeeper you visit often and try to reduce your shyness and anxiety.