Every yes is a no to something else — you just don't see the thing you're refusing, because it's invisible at the time. The evening with your family. The work that mattered. The rest you needed.
We say yes to look generous, to dodge the small discomfort of declining, to be the one who can be counted on. The bill arrives later, in a week with no room left in it.
Saying no to a good thing isn't unkind. It's how you protect the few great things. A yes you'll resent helps no one — least of all the person you said it to.