Do What You Say You Will Do

I am extremely frustrated right now. If I ask you to do something and you do not want to do it, simply say NO!  Why do people always agree to do things they don't want to? Is it to preserve a friendship or you just can't say no to just say people? I hope not to preserve a friendship cause really when you don't go through on the agreement the friendship is definitely out the window. Its a waste of my time! Do what you say you will do,after all; ITS YOUR WORD! Is that too much to ask??