I think they at least began as "true believers". (I was.) At some point they may have developed doubts. (I did.) It's amazing how you can reason away the doubts. (I did.) "It's the closest thing to the Truth." or "There might still be some small errors in understanding but Jehovah will correct all that." "Even if it is wrong, it's still the Best Way of Life!"
In the specific case of the GB, they are surrounded by "Yes Men". Everything they do is "worshipped". No one DARE disagree as that would be like disagreeing with God.
I suspect they see their comfy (or even lavish) lifestyle as evidence of God's Blessing. Everything good is a reassurance that they are on the right track. Everything bad is a test from Satan.