Okay! now let me think through this; It's sometimes very difficult to say no to some people even when the play isn't in our favour. Of course there are people who have had a huge influence in our lives, and one day they come up with a little favour from you, and their demands aren't really to your favour, you will definitely find difficulty in saying "No".
But to the vast majority of people, it is very necessary to say "No" politely.