The challenge is to resist circumstances. Any idiot can be happy in a happy place, but moral courage is required to be happy in a hellhole.
It's always a challenge to discover the most effective first sentence, and the most effective final sentence, in a chapter for instance, and in the book as a whole.
We are stimulated to emotional response, not by works that confirm our sense of the world, but by works that challenge it.
I write in longhand and assemble lots of notes, and then I try to collate them into a coherent chronology. It's like groping along in the dark. I like writing and find it challenging, but I don't find it easy.
Though I am never exactly "blocked" I do have difficult periods. I am led by a fascination with material - the challenge of presenting it in an original and engaging way. I have no problem imagining stories, characters, distinctive settings & themes - but the difficulty is choosing a voice & a language in which to present it.