It really is often not the student's fault when they're acting out. Crazy behavior in the classroom can so easily mirror crazy circumstances at home.
More often than not, teachers learn this lesson the hard way: through parent teacher conferences, phone calls, or other events in which they are unprepared to meet the source of crazy, erratic, or disrespectful behavior.
When we are little, we totally swear that we won't make the same "mistakes" our parents made raising us.
But then the time comes to be parents ourselves...and it's not as easy as we thought it was in the first place. Nothing gives you sympathy for your own parents quite like having to become a parent in your own right.
Which is why sometimes, it's hard to keep those promises we made to ourselves.