When do we learn to say NO?

We rarely ever say no directly unless we are angry or frustrated. The thinking men and women ought to have the courage to say it as it is. But we don't. We have politicians who on one hand say that they are the protectors of our city and they are the same people who also tend to destroy it at the slightest controversy. We have internal and external enemies who are creating ruckus and we cant tell them no we have had enough.

We cant say no to our friends when we don't want to do something. We think of excuses to make them feel that we are busy or we are up to something. Why cant we say no and why cant others respect the no.