Why do cab drivers feel obligated to share words of wisdom?

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I kid you not. I've yet to ride on a cab with a driver who doesn't fill the urge to spill his entire life story on me, or preach against the eroding values of teenagers. Why are cab drivers such a talkative lot? Is it because they feel that for at least 15 minutes or so, the passenger is their prisoner and they can yak on and on and not be told to shut up?