Human beings have evolved to cooperate with each other which is why our dominant rule of thumb is the rule of tit for tat.
We are wired to cooperate and help a fellow human being unless that other human being refuses to cooperate with us. In such a case, we retaliate. Tit for tat is a relatively good strategy and can be said to be optimal in some strategy games.
But there is dark side to employing this strategy. We have developed a habit of saying "yes" to a request without really assessing the opportunity cost of agreeing to do something for someone else so much so that some people can spend their entire lives just trying to please other people.
So Charlie Munger of the Berkshire Hathaway fame suggests giving the "five-second no". He would think about a random request for five seconds and then just say no. Some people might be disappointed at rejection but Charlie found that it seldom hurt him over the longer term. Instead, people knew where he stood when it comes to the use of his time.
The book does not consider the middle ground which was amply explored by Mark Manson in The Subtle Art of Not Giving a Fuck.
As a trainee in a law firm, the value of my time is negligible. As I don't have a lot of social capital in this new alien industry, I can't really say no to a lot of things. In fact, I have to say yes and then do my best to meet whatever requests that come my way. If a trainee eventually rises to the position of a partner, his time may be worth thousands of dollars per hour. His opportunity cost is so high that he would have to say no to many things unless the other person agrees to be billed for his time. ( In fact, he might even have to say not to his own family )
Whether we can say yes or no largely depends on how much we need the cooperation and support of the folks around us.
If we exist in an environment where there is a lot of interdependence, then we should always say yes in the hopes that others will help us when we find ourselves in need. In an environment where we experience a lot of independence ( much like financial independence ), it is much easier to just pause for 5 seconds and then say...