As others have said, it's a pretty dreary world they're in, and they're fighting to overthrow an all-powerful Emperor-God... it's never likely to be a walk in the park for them, so it shouldn't seem particularly nice to the reader either.
There is most definitely a lot of struggle, and quite often the struggle is for the characters to find optimism and reason (which I admit, could be pretty darn depressing!) but at the same time, it puts their struggle into good context. If any of them were more cheery about it (okay... so Ham and Breeze tended to be relatively cheery) then it would make it seem as if their struggle wasn't as great as it was.
In short... yeah, it's fairly depressing for quite a bit of it, but if it wasn't, then the tone of the book would be at odds with the events of the book I think. It's a bit of a necessary evil.
There is most definitely a lot of struggle, and quite often the struggle is for the characters to find optimism and reason (which I admit, could be pretty darn depressing!) but at the same time, it puts their struggle into good context. If any of them were more cheery about it (okay... so Ham and Breeze tended to be relatively cheery) then it would make it seem as if their struggle wasn't as great as it was.
In short... yeah, it's fairly depressing for quite a bit of it, but if it wasn't, then the tone of the book would be at odds with the events of the book I think. It's a bit of a necessary evil.