This is a very interesting question. There might be a line of logic which suggests that part of the reason that so many of those who entered the camps maintained such a positive attitude was caused by a sense of denial. The notion here is the idea that individuals who kept a positive and strong demeanor could have done so in the idea of denying the truth of how bad things really could be. This could have been a defense mechanism because the full grasp of the horror that awaited was far too awful to contemplate and to internalize. The belief that "it cannot be so bad" or "they would not do that to us" or "how do we know this is going to happen" are all examples of the denial that is present in the thinking of those who were subjected to the camps. At the same time, I do not think that it was all denial. There was a sense of religious faith that many clung to in this difficult time and within the idea religion and redemption, there could be a sense of positivity towards their predicament.
The Germans were also very systematic in their approach to the genocide that was the Holocaust. Part of that approach included misleading the Jews until the very last possible moment. They were told they were being "resettled in the East", and "Arbeit Macht Frei" work would set them free. They were told they would be taking showers when they arrived in the death camps, when in fact many were being gassed instead.
There is a very strong human tendency not to wantto believe the worst in possible, and the Germans very systematically gave them an option that seemed more hopeful.
Add to this the fact that Jews had been persecuted in Europe for 600 years by that point, and while it was horrible to endure, such persecution had always eventually passed. Jews wanted to believe this was just a difficult period that would get better if they just waited it out.
In the book Night the Jews to be deported first were the foreign Jews. The Jewish people who had ancestry in the country had no idea that Germany was going to harm them. Many of them had fought in World War I and many others had held high positions in government.
It was also not conceivable to the average human being that mankind could be capable of such horrible acts against another person. They were also consistently lied to by the soldiers who told them they would be moved to a new place for safety or so they could work and that they would be alright.