Tolstoy once came down into his living room from the office he wrote in, saw his family laughing, and said, "How can you be happy when Prince Andrei Nikolaevich is dying upstairs?" When you truly experience that feeling concerning your own characters, you are a writer.
Post-novel depression's intense pain tortures the most talented writers. When the writing is finished, the developing, changing, coming-to-life characters are no longer alive and growing. They are the writer's children during the writing process. When the novel is done and the writing stops, they become unchanging memories. It's like your own children have died, and all you have left is their memories.
David Brin says that writing is the ultimate sadomasochistic experience. A writer succeeds when the readers can't stop reading, giving up food, sleep, and sex, because they can't put the book down. The greatest compliment I ever received was when an MIT student complained that my novel gave him a bad grade on an exam. With the best novels, readers are in bondage to the novelist. The aim of the writer is to create characters as vivid as one's best friend, parents, or even spouse, so that the reader worries terribly about what is going to happen next to that character. The readers are enthralled, with "thrall" being another word for "slave." But the one most enslaved by the writing is the writer himself, as Tolstoy was with Andrei Bolkonsky. Tolstoy destroyed my vision, because I read War and Peace in one weekend.