For me, there were a whole host of things that came with being uncomfortable saying no, such as: building resentment resulting from doing something I didn’t really want to, passive aggressiveness, avoidance of conflict, disappointment when people still weren’t pleased after doing what they asked/shifting goalposts, feelings of being and/or actually being taken advantage of, etc.
Pretty much all of the related issues above caused issues in relationships and anxiety, among other health issues.
On the messier side, a few years after our divorce she ended up suing me to adjust our divorce settlement and as part of that process gave a deposition in which she gave no legally relevant details but instead offered a diatribe on what she considered my failures in our marriage. (This did not impress the judge.) It was surprising to me for a few reasons. First, our divorce had been amicable. Second, she had raised none of these issues while we were married so we could have talked about it and/or fixed the issues. Third, it showed that she not only didn’t appreciate the tremendous effort I put into the marriage and her and our family, she wasn’t even aware of it.
Some portion of that is on her, but by never saying no I definitely helped to normalize a very high expectation as “bare minimum” or “no big deal”. I definitely felt unappreciated during the marriage but I just kept trying to do more. And if I don’t appreciate the work I do... why would anyone else?
Simplistic summary: Just because you are "throwing" love doesn't mean someone is "catching" the love that you are throwing.
It explains a lot where someone may wash their partners car (Act of Service) but the partner complains about never receiving flowers (Gift Giving).
This concept was incredibly foreign to me until it was named, and then it became incredibly obvious, and not limited to relationships.
I basically have two answers to requests - it’s either “sorry, gonna pass” or “HELL YES”.
Edit: A trick I learned from Non-violent Communication is to reframe requests internally from “I have to do x” and “I should do x” as “I want to do x”. It much easier to make a decision when you hear the latter out loud.