For much of my professional life, I've been a people pleaser.
It's only in the last handful of years that I began to realize how, by saying "yes" to work that I didn't feel completely aligned with (in order to please someone else), I was spreading myself too thin, I was perpetually busy, and I was essentially saying "no" to my own dreams and desires.
That explained why I would often feel resentful towards the other person for putting me in that position. But really, I resented myself for not having the courage to simply say "no."
While saying "no" is still sometimes hard, I'm now able to do so more diplomatically and regularly than before--and often without feeling the need to give an explanation, which I believe is the next step in the evolution of honoring one's self.