Now that you ask the question, Virginia, this seemingly simple question challenges "formerly irrefutable" arguments; pessimists were seen as negative, "painting our world all in black", while optimists were more seen as positive, never giving up.
But there is no balance if "optimism" turns a blind eye on reality, cautiousness and rational considerations.
They represent Yin and Yang, the "dualities", which are opposing each other, but complementary and interdependent, depending on each other to keep the balance.
According to Albert Camus in his philosophical essay "The Myth of Sisyphus",
"There is no sun without shadow, and it is essential to know the night."
But "knowing the night" does not mean that there is always night ...
That is where optimism reminds you that
"The darkest moment of the night is just before the dawn" (or "came just before the dawn")
and that we should never lose hope.
On the one hand, we should not give up to support our values defending sustainable and more equitable systems and social structures, but also accept to listen to our conscience and reconsider misunderstood ideas, irrational superstitions and fears, or erroneous opinions.