In the case where I said yes, the favour being asked of me was something I was rather uncomfortable doing for personal reasons, and I was being asked it in front of a lot of people to whom I really didn't want to have to explain why I didn't want to do it. So I said yes, thinking that I would just grit my teeth and do it anyway, but at the same time sort-of-knowing that I might not.

Afterwards, I realised I didn't care enough aboout what the asker thought of me to make me actually grit my teeth and do it.

If it was just something like lending a book, and I didn't want to lend it, I would just say no!