I do have a bit of a problem saying no, largely, I think because I 'respect' authority too much. For example, right now I am the only person working on a series of projects, the sort of which an entire team normally handles. But now everyone's depending on me, so it's too late to back out.
However, I say no a lot more than I used to, and it is amazing.
I, being an idiot who says volunteers for everything, volunteered for it despite everything else going on. After two days, I managed to cram the first three subjects out of 20 or so, truly comprehending very little of it. At that point, I realized that it just wasn't going to happen and told my boss that I wished to withdraw. His response was something along the lines of "That's probably a good idea." It didn't really feel like anything.
As a second, quick, example. Someone just asked me to critique a lengthy excerpt of her novel. This was very easy to say no to, as it was erotica, which I cannot stand.
Estimate how long it will take, tack on 25% to account for planning fallacy, compare to current schedule and priorities. Another recent thing I said no to would have required at least 4 hours a week for two months, in addition to at least $500.
Both? Thinking about other people suffering is one of my main motivators, but I have trouble feeling anything for people on an individual level.
Yes. Without going into detail, I said 'yes' a few too many times and reaped horrible consequences. Also, it's a lot easier to say no when you primarily associate with people you don't like.
Some people seem to be a bit too generous for their own good. I know a precious few people who are especially good at saying "no" when asked to take on new responsibilities that would put them over their limits. I love working with people like that because I can always trust them to tell me when it would be better for me to find someone else to do the thing. I expect this to be an extremely valuable skill it would probably be good for many of us to understand, learn, and be able to teach to people who really need it.