In a world where saying yes is being nice, and being nice is being liked, and being liked is important.
In a world like that: Saying no means to negate another. Our gut reaction is to never do that, to never hurt another person like that.
In a world like that: We have to learn how to say no. To say a clear "no", free of any obligation to explain that no. Just a kind to yourself first, "no".
So that we can live in a world where we all can trust that a yes is truly, a yes.