Jehovah's Witnesses are very concerned that if they associate with nonbelievers or consider opposing viewpoints their faith might be weakened. There is good reason for this.
When JW's say they have 'the truth', what they really mean is their beliefs are based on faith. Even though their faith makes perfect sense to them, and they can 'prove' it to themselves based on their understanding of God, the Bible, or other factors, they know that others can disprove what they believe using similar logic.
Many JW's don't want to consider what critics say. They rationalize that it's a waste of time, they already have 'the truth', what can someone 'out of the truth' teach them, ... etc. , but there's another factor that's much bigger and far more scary to JW's. What if they examine their religion and come to the conclusion they are wrong?
Jehovah's Witnesses have much more to lose if they decide to leave. Recovery from their religious programming can take a few months, years, or a life time. Some never recover. JW's who ask a lot of questions and are open minded can find happiness fast. Others in never achieve true happiness outside the organization.