Why is it that Antoine and Roquentin and Mathieu, who are...

Why is it that Antoine and Roquentin and Mathieu, who are me, are indeed so gloomy? – whereas, Heavens!, life for me isn’t all that bad? I think it’s because they are homunculi. In reality, they are me, stripped of the living principle. The essential difference between Antoine Roquentin and me is that, for my part, I write the story of Antoine Roquentin [..]
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I stripped my characters of my obsessive passion for writing, my pride, my faith in my destiny. My metaphysical optimism – and thereby provoked in them a gloomy pullulation. They are myself beheaded. And since one cannot touch a synthetic whole without causing it to die, those heroes are unviable. I hope they aren't entirely so as imaginary, fictional creatures; but they can exist only in the artificial milieu I've created around them to sustain them. Apart from the sadness of disintegration which I just mentioned, they have another still deeper kind: the sadness filled with bitterness and reproach of Homunculus in his jar. They know themselves to be unviable, sustained by artificial feeling – and insofar as the reader constitutes them with his time, he feels pervaded by the metaphysical sadness of prehistoric animals doomed to imminent extinction  by the inadequacy of their constitutions.
Sartre, War Diaries. (via)