I think it often comes down to a craving for significance. Understanding things no one else does is very intoxicating.
Consider C.T. Russell as an example. I think Russell really believed the Adventist date calculations based on numbers in the Bible and his own additions to them.
For example, despite the fact that there is no reason that the Great Pyramid's dimensions in the modern measurements of "inches" should mean anything at all, he became convinced that they verified those dates and that there was no way this could be a coincidence. If you look at his published charts, he came up with numerous symmetries in his date system (era X before Abraham exactly equal to era Y after Abraham, etc.). The more he was able to come up with these "patterns," the more convinced he became that God was revealing these things to him.
As his writings spread (which he saw as proof of God's blessing), various clergy and Bible scholars started warning people that his theories were nonsense. This opposition convinced Russell that they were jealous Pharisees and that he was God's Laodicean messenger, a prophet crying in the wilderness in the last days and battling those false religionists on behalf of God.
Other people began following him as God's spokesman. After that, there was no turning back. He really believed his own hype.
I go back and forth regarding Rutherford. I tend to think that he knew he was a ruthless manipulator and just cynically used the religious trappings as a scam to power and control. Then I read his "jubilee cycle" explanation of his 1925 prophecy in the "Millions" book and I start to think that he really considered himself as God's chosen successor to Russell.
Fred Franz seems to me much more like Russell -- clever at finding meaningless patterns in the Bible or history and truly believing that God was revealing hidden truths through him.
Based on Ray Franz's statements in CoC, I think the later GB members don't see themselves as God's messenger the way Russell did. Rather, they hold fast to the concept that they are part of God's chosen organization. They really believe the "faithful and discreet slave" concept, the idea that the organization is something greater than them. Rather than coming up with new date systems like Russell, Rutherford, and Fred Franz, they are busy plugging holes in the dike, trying to come up with new explanations ("generation") to keep the whole thing going despite the failure of so many of the predictions. They aren't as sure as Russell that their explanations come from God because they have seen so many such failures. Their favorite scripture is Proverb 4:18, because the "progressive light" idea allows them to maintain their belief in the "chosen organization" concept despite all the evidence to the contrary.
So many have based their entire lives on this organization that the possibility that the whole movement is based on a delusion is simply more than they can bear to contemplate. They are determined to grit their teeth and endure to the end because their entire self-concept is based on their being "in the truth."
