The main difference between yes and no is that one is easy and the other hard.
Yes can be said by any old fool, while no requires character, commitment and courage.
I have also learnt never to give reasons, as they can be challenged, resulting in capitulation. In writing this I have learnt a third thing: never say you cannot do something because you are too busy.
No one will be impressed: being too busy simply proves you are not good enough at saying no.
-Lucy Kellaway ‘Why the most successful people just say no‘ Financial Times