From another post on a similar subject:
Social interaction in the WTBTS flows back into the Witness practice of creating an insular, isolated community. The congregation is your family, more so than your blood relatives. The congregation is your support. The congregation loves you more than the parent or wife or child who is an unbeliever. The congregation has your best interests at heart, worldly friends will tempt you into sin and death. They justify it by stating that Jesus said he would come as a sword to divide the family, and then shift the blame by stating that the rift is caused by the actions of the unbeliever (Wt 1/15/75, p. 29).
The goal (with varying degrees of success, obviously) is to create a dependancy on the congregation. Once social ties outside of that are weakened or lost, it makes it significantly difficult to leave because there's nowhere to go and no-one to go to. That isolation and social dependancy is one of the primary tools used by fringe cults to indoctrinate and maintain followers, because it creates a circular, self-sustaining thought process - the outside viewpoint that the Witnesses are wrong is taken by the Witness and used as evidence that it is right, and any stife or attempts by friends and family to get the person to leave feeds back into the conviction to stay.