I don't usually say "No" to people.
Because it is easier to say "Yes", just do it and then, get over it (especially if it comes with some unwillingness).
But there comes a day when saying "Yes' is putting more strain than ease in a relationship, simply because I am tired of giving in to the demands all the time.
And so I ask myself, "What is the point of saying "Yes" reluctantly only to find myself with a stronger desire to avoid this person in future?" Saying "Yes" in this case, is more destroying than constructive. We should strive to build healthy relationships, else - don't start any. Keep all contacts polite, formal and distant.
Most importantly, it's about knowing what you want, not being afraid to say it (And "No", if it's NOT what you want) and expressing yourself.
And so, I'm learning to say "No". It saves more regrets than I could have avoided.
.
.
.
.
Don't be too quick to say 'Yes'; sometimes, saying "No" will only make you more humane.