This might be true for an extreme pessimist/optimist. Why do you ask? It's just a definition, available from any dictionary.

I totally agree with you.

In extremis, pessimist has frustation, envy. He didn't understand the really meaning of life: a bit of dream, a bit of madness, strongness, patience and passion (things than only optimist sees)

It's like any pithy saying -- there is some truth to it, but the reality of real people is a lot more compliex.

For the pessimist, I totally agree, except if it's more realistic I think. But for the optimist, in my point of view, it could show that they don't want to see the difficulties, maybe to protect themselves. I learn about the philosophy of the optimist (Leibniz) and about the critic that Voltaire made about it. And it was really interesting but I don't know how to translate in English, maybe something like "The world isn't good, the world is the best world where is possible to live".