Saying no helps eliminate future undesirable tasks, freeing up time for what we'd rather be doing.
In the Psychology Today article aptly entitled No Is A Complete Sentence, Dr. Cytowic advises: "The way to refuse to do something one is not obligated to do is to refuse: 'No thank you,' 'No, I’m so sorry.' 'No, I simply can’t.' 'Thank you for asking, but I’m afraid it’s impossible.' Repeat these phrases as often as necessary to wear out the attacker. It is a tactical mistake to ever give a reason for declining. . . .When you offer no excuse you imply that you’d love to participate or oblige if only it were possible."