Someone can be defined "insane" when they have perception of reality that is different from the commonly accepted norm. (This is of course a popular definition, not a clinical one.)
Translation: You are nuts when you see pink dragons and no one else does.
But you are of sound mind when the your judgements make sense to those around you.
So if you are insane, the cure is to surround yourself with other insane people who claim to believe the same things. Voila! you are now sane. In fact, everyone else is insane. This is the good part: now you can disregard everything that doesn't match your perception - because they're crazy! If you begin to doubt your belief, you can go back to your group and reaffirm your "reality."
Now lets compare to JWs. They canvass the world looking for "sheep-like" ones (define as: ignorant, victims, unhappy with their reality). "We have a better reality for you, and we can prove it - our worldwide unity." Now that you are one of us, you must go to all the meetings, and recruit new members. Remember the world outside is insane and in the control of the Devil. Don't make any friends outside this group, they are insane ("Satan has blinded the minds of the unbelievers"). If someone tells you what we believe is not true, don't listen to them. Apostates are angry and vindictive (insane?).
What happens when you meet someone and realize they are crazy? Does that condescending smirk appear on your face as the schizophrenic tells you about pink dragons? Does your mind shut off the possiblilty of reasoning with this person? Have you recognized the same reaction from JWs when confronted with reality?