I know a lot of people, mostly but not entirely women, whose default answer is Yes. If anyone asks them to do something for them, be involved in an event, coordinate details, give them a ride, help them out with some cash, they say yes. Sometimes they have elaborate rituals of being able to say yes without putting themselves at risk, like my friend who will loan anyone $20 if they ask for money. He figures the $20 is small enough that it isn't a burden if he never sees it again, and then if the same person comes back and asks for money again, he can ask about the repayment of the $20 and they tend to leave him alone.
I have friends who really fret about not being able to say Yes. They want to say Yes, you can crash on my couch for as long as you need it, Yes, I can loan you your rent money this month, Yes, I would love to buy a ticket to this awesome event and go with you, but circumstances prevent them from being able to actually say the Yes. It makes life hard for a single mom family to have a person staying rent free and abusing the thermostat with no move out date. It is impossible to loan a month's rent to someone else when you don't know if you'll be able to make your next rent payment. And if you don't have the money to spend on an evening out, then you don't have the money to spend on an evening out. It's a boundary thing, it's a people-pleasing thing, and often it's a I-have-to-earn-your-love-because-I-am-not-enough-in-myself thing.
In contrast, my default answer tends to be No. Surprise me, and I will probably say No. If I am under stress, or the suggestion sounds hard, or I don't know where the money to lend would come from or whether you will be absolutely reliable at paying me back by our specified (and perhaps signed document certified) date, I will say No. I have actually learned to mitigate my No-saying by asking for time to consider, since a flat No can shut the door to great adventures. I will say, No, not right now, or No, maybe later, or No, but let me see how this works out and I'll get back to you. I figure it is much easier to change a No to a Yes (and makes the asking people happier) than changing a Yes to No when they have already begun to count on your involvement.
I first realized my No-saying tendencies when my friend would ask me to add another person to my weekly food distribution. My first answer was almost always No. I would then think about why I said No, whether it was because I didn't want to deliver to another person or I was at full capacity or I didn't want the hassle of finding out whether the new person would be reliable at picking up, and discuss it with my friend. Often, she troubleshot my objections by figuring out a way to get someone else to do the delivery or vowing to find someone else to take the food if the original intended recipient didn't show, and in the end my No became a Yes. But I did wonder at my propensity for saying No to situations that looked like a hassle.
I have been complimented on my boundary setting and maintenance, and I will say that knowing how to say No has a lot to do with that. I also want to add that there is what I think is a separate process of saying Yes to Yourself, which is a way of affirming the self and also, in my opinion, learning to say No to others when saying Yes would mean sacrificing yourself or working against your own best interests and realistic resource limits. I think sometimes saying Yes and saying Yes to Yourself can be mixed up, but that they are quite different.
The solution for bringing it all into balance seems to be asking for time to consider, interestingly enough. Whether the default answer is Yes or No, putting a delay on decision making until you have time to think it over a little bit and really weigh the feasibility seems to be the best way to maximize enriching experiences and minimize commitments that do nothing but drain resources and add stress.