Do you realize that as an adult in this society, everything you do, every minute of every day, is your choice? Nobody can "make you" do anything! Once you realize this, when you say "yes" you own it, and you never do anything half-heartedly again. Even when I make hard choices, I realize that I could have said "no" and I own my actions and do them whole-heartedly.
The more comfortable I have become saying "no", the more vulnerable, generous and unguarded I've become with others. In other words, the more comfortable I am saying "no", the closer I get to other people and the more likely I am to say "yes"!