Staunch, But -
Be just to all, but never trust all. (American)
Patience can be a big key to content. [With Mohammed]
A drop of ink may make a million think. (American)
Placidity has its price. Placid people tend to get invaded, and may be made use of, exploited and vandalised, and so on. Placid people need to learn to take care of themselves and maybe bulwark much better. It may not do merely to get firmer.
Ancients, including Greeks, tackled many problems, built fortified cities and walls and engaged in defending themselves as skilfully and violently as they were up to, and as a result of some freedom had chances of survival. Progressive culture levels were "fruits" of withstanding invasions and preserving one's freedom degrees much, and of the stability of peace. Ancients were at least potentially quick-tempered, for that is natural.
A problem of our times is that dispensing with what is natural, or relevant parts of it, breeds curbing, and slowly this gives rise to diseases of various sorts. Much stress is a consequence of not behaving naturally, but of restraining oneself in stressful conditions over and over, till "the dam" of control breaks, and even more serious problems are formed, including a lot of psychosomatic diseases.
"You can't be too peaceful" has two meanings. And don't get stuck in one of them, or in the middle. However, you can seek a middle way or middle ground some way or other, so that you can function too.