(a) The optimist views the upcoming day positively, and believes something "good" will come of it "somehow or the other." The optimist also believes in some sort of relationship between individual effort, good will, rational prudence, and positive results.
(b) The pessimist has no such positive expectation, denies any rational or just ordering of society or the events that transpire within it, and is fairly certain many bad accidents and other unfortunate developments will result every single day (despite our best efforts to avoid these things). Less dramatically, the pessimist may also simply "see" most days as presenting an endless procession of neutral events (e.g. chores like placing out the trash and buying groceries, small talk at work, paying bills). These "neutral" things aren't bad but they aren't good either; they simply don't have enough value in them to make a day meaningful, enjoyable, appreciable and for that reason can't be considered good.
Optimism often takes human beings and human nature to be "totally good" and denies the laziness, stupidity, animality, cruelty, out-of-this world selfishness, and outright savagery inherent in human nature (man has been preached at, instructed, commanded for several thousand years to rise above these instincts but not a single human generation has achieved such transcendence, even for a single day). The certain belief that something good will happen may cause you to misinterpret and mistake neutral or even bad events as being good.
To be more precise, the specific deception that optimism frequently engenders is the view that humans, society, our nation, you yourself are "progressing" (or are easily capable of progressing if just a few things are done differently) when the facts and information invalidate this assessment. From the mental momentum generated by decades of being optimistic, the optimist can become absolutely convinced things (or the self) have improved tremendously when things may have stayed more or less the same or even deteriorated.
(6) Pessimism also tames your expectations of what you and others can sensibly achieve or will be willing to sacrifice and thereby allows you to live your life not so much "expecting the worst" but able to take seriously possibilities for how things will develop that are sub-optimal. Pessimism has the huge disadvantage of being extremely unpopular these days; to be branded a "pessimist" is to have one's thought, character, personality almost totally rejected and attacked in literally "one word." And pessimism can quickly devolve into depression and a whole host of other mental health issues which we should rightly seek to avoid.
(7) Our position on pessimism is becoming more articulate with the passage of time. We believe that life, reality, Nature, human society present (a) infinite good, (b) infinite neutral, and (c) infinite bad for man to experience. Most people get stuck in infinite neutral and infinite bad. This is the factual material for justified pessimism as to the human condition. While the human condition is not utterly depressing, man does enter the world helpless and departs helpless. In between, many spend their entire lives doing work they despise so they can either buy things to impress people they don't even like or gather fleeting pleasures of the most trivial value, trying to prove their self-worth through their careers, families, or hobbies, obsessing all along the way about what "other people" think of them when, in reality, these other people spend almost zero time thinking about you. Somehow, there is something "here" in this banality for us to be wildly optimistic about??!?
Human life is time finite (and, even worse, uncertain as to duration) and so one's choices should gravitate as soon as possible toward the good and away from the neutral and the bad. For these decisions to be abiding, there must be some emotional fuel continually reaffirming the decision of the good over the neutral and the bad. When one is presented with sufficient facts, information, and reasoning to justify some human reality as either neutral or bad, then the job of pessimism immediately begins. Pessimism now has its "marching orders." Pessimism no longer gives this reality "a shot," does not allow it to motivate behavior any longer, and essentially closes this box forever: "they have gained knowledge and nausea inhibits action...Knowledge kills action...true knowledge, an insight into the horrible truth, outweighs any motive for action..." (Nietzsche, Birth of Tragedy)