Greg McKeown in Essentialism: The Disciplined Pursuit of Less,

We have good reasons to fear saying no. We worry we’ll miss out on a great opportunity. We’re scared of rocking the boat, stirring things up, burning bridges. We can’t bear the thought of disappointing someone we respect and like. None of this makes us a bad person. It’s a natural part of being human. Yet as hard as it can be to say no to someone, failing to do so can cause us to miss out on something far more important.