The following is copied and pasted from www.freeminds.org:
Why Disfellowshiped Persons Often Defend the Watchtower
What a surprise it is for many who encounter a person who defends the Jehovah's Witnesses both in belief and practice, only to find out that they were disfellowshiped and no longer associate with the Witnesses!
One would think that being away from the Watchtower for a time would allow them to investigate the teachings of the WT, and discover its errors. But this is often not the case. Why? Here are a few mind-controlling factors:
Guilt
Many who leave the Watchtower were "practicing sin," either sin as defined by the Bible or perhaps just by the Watchtower (such as smoking or celebrating holidays). The victim's conscience is pained, constantly reminding him of his wrongdoing, and also preventing any objective examination of the organization itself (as to teachings and history). Any effort to examine "apostate literature" or even to go back and read out-of-date WT Literature would be thought of as an attempt to justify their own sins, leading to further guilt.
Fear
Fear now takes control, as the disfellowshiped Witness is convinced that the devil is out to stumble and confuse him even further, especially by allowing doubts about the organization (which is "doubting Jehovah himself"). Fear of punishment from God continues to "protect" the victim long after leaving the WT. Any attempt to critically investigate the WT is squelched by this very effective thought-stopping process.
The Love Affair
This refers to a JW's love affair with the organization. Though it would seem to be over if he has been disfellowshiped, the victim still longs for the sense of camaraderie, the unity and the predictability of the WT organization. Since he had become so dependent on the organization for everything, he will now play the role of the jilted lover, hoping to get his "loved one" back. Since "absence makes the heart grow fonder," every little pain and struggle will remind him of the "mother" organization.
What You Can Do
Any attempt to correct a victim who struggles with the above may be met with instant rejection. They are driven by powerful impulses planted by the organization when they joined. Rather than to attempt to reason with these ones using the Bible, it might prove much more effective to arrange for them to hear the testimony of someone who is an ex-member of another cult, and how they struggled through the same kind of fear, guilt, and perhaps even a "love affair" with their organization. Parallel struggles in the lives of others may be just what is needed to open up their minds.
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When I was in, I was totally sensitive to JW criticism, even though I wasn't a diehard, door-knockin' J-Dub by any stretch of the imagination. I don't think that the "love-affair" analogy could be more appropriate. Loyal J-Dubs, regardless of their level of involvement, defend the WT for the same psychotic reasons a battered wife defends her abusive husband. The husband in this case constantly reminds the wife how good and benevolent he is, and how evil his critics are, and how shameful it would be if she ever gave ear to any of those critics who just don't understand what a really good guy he is and spread terrible lies about him in order to tear her away from his benevolent goodness.