In refusing to say NO to others, even though it's the honest answer, you're presumably doing it to protect them.

Their interests require that you say YES, even though your own interests require that you say NO. Yet there's a contradiction here, isn't there? If your interests are not to be put first--because "selfishness is bad"--then why is it GOOD to put the interest of another first? It's true, you're being selfless by saying YES when the answer should be NO. Yet, in the process, you're advancing the selfishness of the other person--only it's unearned selfishness.

Bottom line: We're supposed to be selfless, because it's "good." Yet in the act of being selfless, we're increasing the selfishness of others. We're expected to follow a moral code which requires another to be bad in order for us to be good.