Teachers like to talk, and that is a problem because if students are not actively participating, they find something else to do and that something may not be constructive and it may not be desirable.
I remember in my student teaching days an exercise where we would plot "TTT" (Teacher Talking Time) and "STT (Student Talking Time) on a chart. It was very simple; every 15 seconds or so we would check one of two columns and then at the end of the lesson produce tables and graphs. Inevitably, teachers spoke the majority or almost all of the class, and equally inevitably they were shocked at how much they spoke.
The point of the exercise was to get us, teacher trainees, to be aware of this trap from the beginning so that we would consciously plan activities which were (a) student-entered and (b) designed to get students actively participating and speaking.
Of course, NCLB and modern teacher training has thrown such considerations out of the window. I am convinced that one of the reasons we have such low student attainment and such low student engagement (leading to high disaffection and dropping out, and/or high disciplinary concerns) is because teachers simply talk too much.