I see it so very often when I read people, and recognize it myself as well, saying 'No'... it's hard. We want to be kind, we want to make others happy and help them. We like being nice, we like to please people and we think that saying 'No' means that we are not going to be liked. We think that saying 'No' is not going to be accepted and sometimes that might be true, but does it actually benefit anybody, if we say 'yes' when we mean 'no'.
It turns out that in the long run, selling your truth for short term likability isn't such a good idea and selling your truth because you do not want to hurt the other person is also nearly always going to end up in more tears for both involved rather then less. It's sabotaging yourself, your life and anybody else involved in every way possible.
I had a lady on retreat a while ago, she was going through a divorce, had been married roughly 15 years and she needed to forgive herself. She had said 'Yes' to her husband when he asked her to marry him, because she couldn't say "No" and leave him broken hearted and she was actually seriously asking me why things had gone wrong!
As we were going through the areas of forgiveness, I asked her if she had thought of forgiving herself. "Myself...?" she said, "Yes, yourself" I replied. "For taking 15 years of your life and throwing it to the wind, knowing it could never lead to happiness... and also forgiving yourself for taking 15 years of happiness away from your husband, as saying 'yes' while it should have been 'no', was never ever going to make him happy. How could it, you weren't fully in it. You've made the poor guy work to make you happy but that was a job he could never fulfill! You took 15 years of his life in which he might have been able to find a wife that actually did fully love him and one that was there for him in the relationship."
She started crying, she thought she HAD DONE HIM A FAVOR... while in reality, she now realized she'd done him and herself the biggest disservice ever! He might have been heartbroken for a year or two, had she reclined, but then he would've moved on... and possibly found happiness. All things he would have to do now anyway, 15 years later.
This might be an extreme example, but experience (and having read a lot of people) teaches me that in reality every YES, that should really be a NO, leads to more pain, empty promises, uneasy feelings and more damage for both parties than you could have ever imagined.
When you say No, when you truly feel it needs to be NO (no matter how hard it may be to say it), you actually say YES to your own truth, you say yes to what serves you and to what serves everyone ele in the end too, which is incredibly healing and freeing. It's a clear energy and clarity gives peace. By saying NO when it's needed you clear the path for what is right to come forward. You also allow the other person to find something or someone more suitable! So in reality you also help them to say 'Yes' to what they truly need, even though they do not realize that at that moment in time.
Saying 'No', really is saying yes, it just depends on how you look at it. Try it when you feel it and see how in doing so you are being kind and respectful to yourself, see how good that makes you feel, and see how once you get the hang of it, all that you need in life starts to rush towards you effortlessly, as well as it does for others once they start being real!