Yes VS. No, where to draw the invisable line?

When we are little we are taught to stop saying "no", at times, maybe even punished. Why is it so difficult for people to say "no" to things that their not happy with? One must learn the importance of saying no, otherwise people will take you for granted (I believe it is OK to take advantage of someone, as it gives them the feeling of being needed, something we all long for).

We must also learn to say yes. Yes to life, and the beauties, splendors, and joys it brings to us. Why do we hide and deny our self the good things in life?

The instances where we say yes or no, are usually opposite, let me explain.

A girl who has boyfriend A, doesn't feel like he deserves her, that he isn't treating her the way she deserves to be treated, so she says no to the relationship, (or talks it out and sees if it'll work out through change).

That same girl has left boyfriend A and moves on to a boy she is highly interested in, and he in her. He has asked her to be his girlfriend, yet she feels inferior to him, that she doesn't deserve him. It would be in her best interest to be with him, as it would better her.

Pin pointing the fine line is next to impossible, as there are millions of millions of different scenarios and such. But I guess you will have to find which works best for you, but either way, just don't sell yourself short, after all, you were born to be more.