Being unreactive to challenging situations is often the strongest option available to you. When a girl is testing you hard, someone is publicly belittling you, or things are just generally going crazy and spiraling out of control, the most nonplussed, nonchalant man typically wins: he shows dominance, control, and unflappability.
However, sometimes the calm of unreactiveness must be set aside, and situations simply need dealing with.
Sometimes the girl testing you needs to be set straight; sometimes person belittling you needs to be put in his place; sometimes the crazy situation requires you to place both hands on the wheel and make things sane again.
If you react in situations where remaining unreactive is ideal, you violate the Law of Least Effort and appear tryhard; yet, if you fail to react in situations where your reaction is sorely needed, you seem weak, fearful, and indecisive.
That makes things necessarily a bit trickier, because there isn’t a perfect one-size-fits-all response to every situation; sometimes it’s better to not react, other times better to react.
To know which one is called for, you must have a read on the specific situation... and you must be able not to bow to social pressure.