It's literally in the exact same book that he writes that those who lost hope died first. Hope is what keeps us going.
He is talking about his companions. If we try to understand the author's perspective, he was somewhere between the spectrum of men who lost all hope and incorrigible optimists.
For someone like that, I believe the incorrigible optimists would be much more irritating. It's like, there are so many problems and instead of fighting them, the optimists don't want to acknowledge they are there.