In the movie Yes Man Jim Carrey says yes to everything after attending a motivational seminar. At first it releases him but later on he discovers you can't say yes to everybody, you have to draw the line somewhere lest you spread yourself too thin or wear yourself out. Common wisdom that, as much as you would like to be the 'nice guy' at some point you have to realise your limitations and just say no without feeling guilty about it. You can't please everybody so at some point you have to say no to people who are well meaning for all intents and purposes.
Conversely I've come to realise that you also have to be prepared to hear no too. It could be in the form of a job that you thought was yours- been there or a relationship that didn't quite go the way you wanted. Some doors have to be shut for others to open and you see that everything doesn't go your way all the time. Rejection matures you and reminds you that you are not infallible or invincible and that these things do happen but dust yourself up and try again.