From the time we are small, we are taught the virtues of obedience and being good, not asking too many questions, especially our seniors, which all translates to one thing in short - dont say 'NO', say 'YES'..obey. However, I don't think, saying 'yes' is always a good way to be. Firstly, if one is saying yes just because they are afraid or conditioned to be averse to conflict, then it is most often seen as being weak and secondly, if you are saying yes, because you dont want to be difficult (which is, I think, a little different from being the first), people still think that they can walk all over you again and again and yet again.. Hence, the difference between being someone who always agree and someone who is assertive, as any social skills instructor would have you believe.
The Problem is to decide when is the right time to be assertive and / or saying yes. I for one, never know. Hence, at times, I appear difficult, at others, a doormat - neither of which is what I want to be...