One organization that practices disfellowshipping is the Watchtower Society. They explain that a total cutting off from contact with members works for them and although harsh it has many benefits. One disfellowshipped person said this, 'Being totally cut off from loved ones and from close contact with the congregation created a strong desire to repent. I realized just how wrong my course was and how serious it was to turn my back on Jehovah.’ The Watchtower 1988 4/15 p. 30 par. 23 Discipline That Can Yield Peaceable Fruit
Can't you feel the intense love for Jehovah and God's organization when you read the above statement? Was a gun pointed to someone's head when they wrote those words? Probably not. Taking away your family, friends, God, and way of life is powerful motivation to 'love' Jehovah with all your heart.
So it seems that the reason for shunning is that by totally cutting off people from their parents, children, brothers, sisters, friends, etc, people are so lonely and depressed that they are highly motivated to swear that they love everything about Jehovah's organization just so they can have all their loved ones back!