A lot of authors are unhappy. I've been reading Faulkner, Hardy, Lewis Carroll, and Yeats, and despite the glitter, they just miserable people.
I think it's related to their view of reality. If life is accidental, nature "red in tooth and claw," and the meaning of life nonexistent, then they are quite right. And we should be miserable too.
"If Christ is not raised from the dead, then we are of all men most to be pitied."
On the other hand...