Even starship captains get the blues sometimes. And when you get depressed, or when you're stuck in bed with a nasty case of black plague, you need something to cheer you the frak up. Luckily, there are tons of science fiction and fantasy books with proven restorative properties.
Everbody loves a good dark, horrible fantasy. A misanthopic adventure, in which everybody is morally compromised, and we all live and die in the dirt. But every now and then, it's nice to read a fantasy novel in which people are, you know... good.