Theodore Dalrymple separates pessimists into two distinct groups:
Pessimists are of two types, the catastrophists, that is to say the types who look up in the starry heavens and see (metaphorically) only asteroids in the sky racing towards us to wipe us out as the dinosaurs were wiped out; and existential pessimists, that is to say those who see dissatisfaction as the permanent condition of mankind because of his inherent makeup, his contradictory desires and emotions, dissatisfaction that is perfectly compatible however with a great deal of enjoyment of life. I am a pessimist of the latter kind.
The former kind of pessimist, those who foresee inevitable universal collapse, destruction, death by epidemic, and so forth, have no sense of humor, or at least of irony. For them, the furrowed brow and the shoulder weighed down by care are signs of intellectual and moral seriousness, the sine qua non genuine concern for humanity and (God preserve us) the planet. Like catastrophe itself, they are not much fun.
The existential pessimist is light-hearted, for he knows that human life is not perfectible, and can therefore enjoy what it has to offer without any sense of guilt that he is not spending his every waking hour averting disaster or bringing perfection about. He does not deny that many diseases currently incurable will one day change their status and that this is a good thing, for taken in the round more life is better than less; but neither does he expect that, when formerly incurable diseases have become curable, human complaint and dissatisfaction will become things of the past.